NOVA

NARRATOR

January 1942. America is just weeks into World War II when the European fight comes to our shores. Hitler's U-boats waste no time going on the attack.

BOB BALLARD

They brought the war to us in a way that caught us by surprise.

NARRATOR

The attacks are devastating. Thousands of lives lost, more ships sunk than at Pearl Harbor, and Nazi spies secretly delivered to American soil.

MARTY MORGAN

They kicked our asses, and yet we were getting no payback.

TIM MULLIGAN

A lot of Americans have forgotten how close the war came to our shores and how close it was to our homes.

NARRATOR

Now renowned explorer Robert Ballard and his team are returning to this forgotten battlefield with the latest technology.

BALLARD

Okay team, 100 meters. Here, we're able to get a picture that shows you what it would like if you could take the water away.

NARRATOR

Their work will help rewrite this chapter of World War II

and bring closure to a 73-year-old mystery

who sank German U-boat U-166? How close did the Nazis come to victory in the Atlantic? (explosions) Right now, on this NOVA/ National Geographic special.

NARRATOR

The Gulf of Mexico, 120 miles off the coast of New Orleans. These peaceful waters were once the setting for a violent, little-known chapter of World War II. Deep below the surface lie the remnants

of a devastating Nazi attack on America

Operation Drumbeat. Just after the U.S. entered World War II, Hitler's submarines, the deadly U-boats, struck hard and fast up and down the East Coast. (explosions) They hunted down and sank the vulnerable cargo ships that were critical to the Allied war effort. And they took the war further, extending their assault all the way into the Gulf of Mexico. Now, explorer Robert Ballard and his crew prepare to investigate this battlefield. To reach the sea floor, they'll use high-tech remotely operated vehicles-- ROVs.

BOB BALLARD

The beauty of the ROVs, these vehicles can stay down for days and days and days. (people talking over each other) Square up on the target and drive over to it.

NARRATOR

The ROVs descend one mile beneath the surface, where the casualties of the Nazi assault still rest. A World War II-era cargo ship, the Alcoa Puritan. In 1942, she hauled aluminum ore vital to America's wartime factories. Her scars are still vivid.

BALLARD

That's a hole, that's a shell hole. So if you could stop laterally and go in and frame that.

RICHIE KOHLER

These are bent inward.

BALLARD

Yeah, you can tell it went in. Yep.

NARRATOR

Joining Ballard is wreck diver Richie Kohler. He's spent decades studying and diving on sunken U-boats.

KOHLER

This wreck is a snapshot of when the U-boats first came to America. This is when the U-boats were not afraid of us.

BALLARD

We hadn't got our act together.

NARRATOR

Not far from the Alcoa, an oil tanker, the S.S. Gulf Penn, 90,000 barrels of oil still trapped inside.

BALLARD

These are in some ways ticking time bombs in the sense that the hull will rupture and you'll have oil come out. Could you look at that wreckage? Could you look at the wreckage before you go too far?

NARRATOR

But Ballard and Kohler's ultimate goal is a pair of shipwrecks resting not far away, one the hunter, the other hunted. This is the S.S. Robert E. Lee, the victim. In 1942, 407 souls walked these decks.

KOHLER

On board the Robert E. Lee was not only passengers, but there were actually survivors from previous German U-Boat sinkings that occurred out in the Atlantic Ocean.

BALLARD

Square on the gun a little better.

NARRATOR

A three-inch deck gun had been added for protection against the U-boat threat.

KOHLER

This gun never went into play. They never had a target. He's locked up, he's in a stored position.

NARRATOR

The attack came with almost no warning. You could almost imagine them standing along the side looking at what they thought was a porpoise... and then it makes a turn and comes into them. It's actually a torpedo. (explosion) Today, the fatal wound inflicted by the German torpedo is hidden beneath layers of silt on the ocean floor. It's amazing that most of the damage is not visible.

KOHLER

Now, we're looking at a ghost ship. I mean, it's obvious that it was abandoned. We can see where the four lifeboat davits were swung out. Within five minutes, these people were in the water fighting for their lives.

NARRATOR

Just over one mile away,

the attacker

German U-boat U-166. Missing for 59 years, her rusting hull was spotted in May 2001 during survey work for a new underwater pipeline. But how did U-166 sink? Who was responsible? And why is she lying so close to her victim? The official record offers little clue. For decades, the circumstances of this U-boat's sinking have been mired in controversy. In 1942, a young destroyer captain, commander Herbert Claudius, claimed credit for the kill. But the official report said Claudius botched the attack. It was one man's word against the U.S. Navy in a case obscured by the fog of war. Ballard and his team believe their technology, building on the earlier survey work, can finally put the question to rest.

BALLARD

It isn't until fairly recently that this submarine's been found. I'm gonna use my technology, and see if an injustice was done that needs to be corrected.

NARRATOR

It's a story that began three years before U-166 was sunk, before America had even entered the war, when Great Britain found itself facing off against Nazi Germany

in one of the most pivotal conflicts of World War II

the Battle of the Atlantic. Lasting from the first day of the war to the very end, the fighting would ultimately claim nearly 6,000 Allied and German ships and span 5,000 miles of ocean. What was at stake was simply Allied survival in World War II. Now at this stage of the war, England was standing alone against the rest of Europe, which had been conquered and brought under Nazi tyranny.

NARRATOR

If the British were defeated, the war would be all but lost.

ED OFFLEY

There was no way that the Allies were going to be able to invade Normandy from Hoboken, New Jersey. It just wouldn't have worked. You needed a launch pad; the launch pad was England.

NARRATOR

But as an island nation, Britain was vulnerable.

Its survival depended on supplies imported from abroad

raw materials, food, weapons. Roughly one million tons of cargo a year, crossing the Atlantic. By 1941, the Nazis were close to cutting off that flow. The key to their success was the Unterseeboot-- German for "underwater boat"-- known to the rest of the world as the U-boat. The Nazis built nearly 1,200 of these deadly machines.

They were meant for one purpose only

to attack. At either end of the U-boat were tubes for launching torpedoes-- 23-foot-long underwater missiles. At its center, the conning tower. Below it, a maze of controls for steering, diving, and surfacing. Submerged, a U-boat's survival depended on its pressure hull. Invisible from outside, these structural ribs and steel plate were all that stood between the crew and thousands of tons of seawater. Like today's hybrid cars, U-boats relied on a combination of internal combustion engines and battery-powered electric motors for propulsion. On the surface, air-breathing diesels could drive up to 18 knots, roughly 20 miles an hour. Underwater, the electric motors took over, but the batteries were a critical vulnerability. They typically lasted less than 24 hours, and the diesel engines could only recharge them on the surface.

MULLIGAN

A U-boat is ultimately a submersible rather than a true submarine. It's not intended to operate perpetually beneath the water. They have to be on the surface several hours every day in order to recharge their batteries.

NARRATOR

And a U-boat on the surface was a target. (gunfire) Despite this weakness, the U-boats were stunningly effective.

MORGAN

The U-boat was a terrifying weapon of war, and it was especially terrifying when it took on unarmed ships that were carrying cargo.

NARRATOR

Commanding the German assault was Kriegesmarine admiral Karl Dnitz.

MULLIGAN

During World War I he commanded a German U-boat. That's where he learns a lot of his lessons. AXEL NIESTL: He could be harsh at times, but the U-boat men considered him a just leader, and that's why they followed him all through the war.

NARRATOR

With the U.S. still neutral, Dnitz focused his fleet on the British ships. Wolfpacks, groups of U-boats ranging from three to more than 20 subs, swarmed and overwhelmed the British convoys. Dnitz directed the Wolfpacks himself via long range radio. Admiral Dnitz was a very hands-on type of manager. He wanted to know where his U-boats were at all times. He required them to call in on a daily basis. The Allies could hear the transmissions, but they couldn't understand them.

The Germans had a secret weapon

the code machine known as Enigma.

MULLIGAN

The Enigma machine itself looks like a very elaborate typewriter that encodes and decrypts the messages as they're sent and received.

NARRATOR

The secret of the Enigma machine was its constantly changing code. With every keystroke, the code rotors that scrambled the message shifted slightly, generating a new code. No word would ever be encoded the same way twice. Every day, the sender and receiver set the rotors to a position listed in codebooks each operator carried. The Allies had secretly copied one of the machines, but without knowing the settings, they had no hope of deciphering the messages. By 1941, the Enigma-equipped U-boats were winning the Battle of the Atlantic. (explosion) England was being starved into submission.

MORGAN

In fact, British Prime Minister Winston Churchill would later say that there was only one thing that he feared during World War II, and it was the U-Boat.

NARRATOR

Then, a breakthrough. In May of 1941, U-110 stopped reporting in. German command believed her sunk. In reality, she had been captured off the coast of Iceland. Her Enigma machine and the code books specifying the settings were shipped back to London. British mathematician Alan Turing and a secret team of codebreakers went to work.

MULLIGAN

The British were able to break the German naval ciphers by the end of May, beginning of June, 1941. That was critical for saving British convoys for the rest of 1941.

NARRATOR

The Germans had no idea. Mathematics, not munitions, had bought the British crucial time. Until December 7, 1941, when the Japanese bombed Pearl Harbor and everything changed. With America officially at war, U.S. ships could now be targeted. Admiral Dnitz seized the opportunity to tilt the battle back in Germany's favor.

MORGAN

He knows that right after Pearl Harbor, if he sends U-Boats across to attack the U.S. Atlantic coast, he knows he'll get a big payoff because the United States won't be ready.

NARRATOR

Dnitz believed his U-boats could cut England's lifeline of vital cargo by striking at the source. Operation Drumbeat was born. But with his U-boats stretched thin, Dnitz had only a handful of long-range boats, known as Type IXs, available to send to America.

MULLIGAN

He assigns six Type IX U-boats, and then one of them falls out, so it's only initially five Type IX U-boats to operate off the U.S. East Coast.

NARRATOR

The trip to America took three weeks. British codebreakers warned the U.S., but in the wake of Pearl Harbor, the Navy had other priorities.

OFFLEY

Neither the U.S. Navy nor the Army air forces was prepared. Not only did they not have the airplanes and the ships available, but the ones they did have available did not have the sensors or weapons that could destroy a U-boat.

NARRATOR

Even basic protective measures were ignored.

HICKAM

You gotta remember, all of Europe is blacked out now, and all the lights were on. There seems to be no alerts whatsoever.

NARRATOR

Officials worried a blackout could disrupt commerce. But no blackouts meant cargo ships were perfectly silhouetted against the bright lights. They found, in effect, U-boat Disneyland. It was wonderful. Horst von Schroeter was watch officer on U-123.

SCHROEDER

We closed the shore within, say, two or three nautical miles. We smelled the forest ashore, and we saw the autos, the cars, running on the shoreway.

NARRATOR

Erich Topp was commander of U-552 and the third-most successful U-boat commander of the war. It was a shooting of hares. It was because the Americans at that time had not developed counter- measures against submarines, and so we had a very easy game there. It took one week, as I remember, only. One week all the time, and we sank, I think, about ten ships. (gunfire)

MORGAN

In fact, they return to their ports in the Bay of Biscay coast of France empty, having fired all of their torpedoes.

NARRATOR

By February 6, barely three weeks after the first U-boats arrived in U.S. waters, they'd sunk 25 ships. Spurred on by the success, Dnitz sent every U-boat he could spare across the Atlantic, including the newly built U-166, the wreck Ballard is closing in on. Onboard the Nautilus, Ballard's team preps the ROVs. The primary unit, named Hercules, is equipped with several high-definition cameras. Its sophisticated sonar will locate and map the wreck site. This technology will offer a view of the U-boat with stunning clarity and detail. Hercules can dive as deep as 2.5 miles, putting U-166 well within its reach. But just like human divers, it doesn't go alone. Hercules is tethered to a second ROV named Argus. Physically linked to the ship above, Argus acts as a stabilizer and light source for Hercules. It also sends video up to the Nautilus so the crew can watch for potential hazards. Hercules is kept on a short leash to prevent its tether from getting tangled. Operating the ROVs like this is tricky. The ship's computers must maintain its position directly above the wreck so the ROVs can safely navigate the site. So computers are driving the ship. Humans aren't driving the ship right now, it's being driven by computers. As we move the ship, we're moving Argus. And Hercules's job is to stay out on point. It will take over an hour for the ROVs to reach the wreck of U-166. By March of 1942, Dnitz was sending nearly half of his combat-ready U-boats to attack America. The Germans had also regained a crucial advantage, a new Enigma machine.

MULLIGAN

The German navy increasingly suspects that their cipher machine has been compromised, as indeed it has been.

NARRATOR

Dnitz upgraded the Enigma machines on board his U-boats with an additional rotor. Overnight, the new machine, nicknamed Shark by the Allies, made their codebreaking useless. NIESTL: From then on, the Germans were well ahead in terms of information because the Allies no longer could read what the Germans were planning. For Dnitz and the U-boats, the hunting was almost too good. They kept running out of torpedoes and fuel and had to travel 3,500 miles back home to resupply.

Until German engineers came up with an inventive shortcut

the Type XIV U-boat, dubbed the Milk Cow. The Milk Cows were not designed to fight. Instead of carrying offensive weapons, they just carried extra fuel and provisions and even extra torpedoes for the U-boats.

NARRATOR

With 430 tons of fuel and supplies, the Milk Cows were floating gas stations.

And the Germans had an ideal location to deploy them

the Atlantic Gap.

MORGAN

Aircraft based in North America, aircraft based in Iceland or Greenland or in the United Kingdom, their umbrella of coverage left a gap right in the middle. That mid-Atlantic gap was an area where U-boats knew that they could operate with impunity because no Allied aircraft could reach that gap.

NARRATOR

The Germans seemed to hold every advantage. The U-boats were virtually unstoppable. By April of 1942, three months into Operation Drumbeat, nearly 250 Allied ships had been lost in American waters. Not a single U-boat had been sunk. NIESTL: Happy times, they called it. In Germany, "Die Glckliche Zeit." There were more targets available than they could cope with. And the U-boat missions were only becoming bolder. In June 1942, the Nazis carried out one of their most daring attacks on America. Known as Operation Pastorius, the program used U-boats to land teams of Nazi spies on American soil.

MORGAN

The overall plan was that German saboteurs and spies would attack vital railroad bridges, aluminum plants that were making the skin of the aircraft that would ultimately drop bombs on German cities. And the U-boat contribution to this mission was vital. It couldn't have been done without them.

NARRATOR

Two teams of spies were landed, one on Long Island, New York, the other near Jacksonville, Florida. The spies dispersed into the civilian population and began their preparations. Meanwhile, Dnitz and the U-boats kept probing American defenses.

They soon identified an enticing new weak spot

the shipping lanes in the Gulf of Mexico.

MORGAN

We had lapsed into the thinking that everything's fine in the Gulf of Mexico because the enemy is not there. And so Dnitz sends them into the Gulf of Mexico, realizing that he'll get yet another big payoff with the Gulf of Mexico, and oh, God, what a payoff he got. (explosions) In the month of May 1942, he sank 41 ships. That's more than one ship a day going down in flames in the Gulf of Mexico.

NARRATOR

It was to this hunting ground that U-166 would be assigned for her first patrol in America. Home movies recorded by U-166's captain, Hans-Gunther Kuhlmann, show the young crew training just weeks before the mission. On June 17, 1942, they left port and set course for America. They would never be seen again.

BALLARD

There's bottom.

KOHLER

There you go.

NARRATOR

Aboard the Nautilus, Bob Ballard is trying to understand their final moments.

BALLARD

What's it say? Target bearing?

KOHLER

Target bearing is two thirty. Yeah, so it should show up on sonar. They're coming down close enough.

NARRATOR

After reaching American waters in July of 1942, U-166 quickly sank three ships and was looking for more off the coast of Louisiana. On the afternoon of July 30,

another target appeared in her sights

the Robert E. Lee, making its way to New Orleans. U-166 launched a single torpedo. (explosion) The passenger ship never fired a shot in return. Within minutes, it was sunk. But what U-166 didn't realize was that the Robert E. Lee was not alone. She had a naval escort ship, PC-566. Its captain was a young naval officer, Herbert Gordon Claudius.

GORDON CLAUDIUS

He was a farm boy from Nebraska, and he, I guess, wanted to get out of Nebraska. And then of course, his first ship in the Navy was the PC-566.

NARRATOR

PC-566 had been commissioned only the month before. Claudius and his crew were about to see combat for the very first time.

MORGAN

As soon as a German torpedo struck Robert E. Lee, Claudius swings the PC-566 into action. One of the men on deck of PC-566 observes a periscope in the water. Claudius then turns the vessel toward that periscope sighting. He's doing that in an attempt to approach from its blind side.

NARRATOR

A U-boat's periscope has a narrow field of view. With the U-166's lens pointed at the sinking Robert E. Lee, the German captain couldn't see the Navy ship coming.

BALLARD

He's in the fray right now. He's trying to kill this guy. He wants to sneak right up on top of that guy so he can't get away.

MORGAN

He's bearing down on it as fast as he can, and when he's 120 yards away, he sees the periscope retract. (shouting in German)

BALLARD

He's following the wake, and directly over the estimated position, he's setting off a sequential series of five depth charges.

NARRATOR

The depth charge is an underwater bomb and the Allies' primary weapon against U-boats. The attacking ship estimates the location and depth of the U-boat, then sets the charges to detonate when they hit that depth.

COMMANDER

Fire two. Fire two. Water pressure triggers the explosion, sending a shockwave ripping through the ocean. (explosion) The explosion isn't powerful enough to blow up a U-boat. The idea is to simply crack its pressure hull... (shouting in German)...then let thousands of tons of seawater finish the job. But depth charges aren't necessarily a death sentence. The shock wave is only dangerous up close. If the U-boat can dive out of range, she can escape. U-boat crews had a series of carefully rehearsed procedures for trying to elude depth charges, as shown in this actual wartime footage. The bow planes were set to maximum angle, driving the boat deeper. The electric motors were set to full power. (shouting in German) Men who were not on duty ran forward. Their weight helped point the boat down. The emergency dive was the most critical test a U-boat crew could face. Seconds meant the difference between life and death. Horst von Schroeter was watch officer on one of the first U-boats in Drumbeat, U-123. From the order alarm, it took 30 seconds to disappear from the surface and another 30 seconds to be on a depth of 60 meters. 30 seconds can be a long time in war.

NARRATOR

Werner Hirschmann was chief engineer on U-190.

HIRSCHMANN

I would call anybody who was not scared for his life under those circumstance a liar, because it is a scary experience to hear a depth charge dropping into the water and then expect, in about five or ten seconds, an explosion to go off. There's nothing you can do. You can just sit there and wait, and this period of lack of activity is really unnerving.

NARRATOR

The crew of U-166 left no record of their final moments. They didn't send a radio report that day, or even a distress signal. All that is known comes from the report filed by Claudius. He dropped a second round of depth charges. (explosion) An oil slick spread on the surface. So now the periscope's no longer visible, the depth charges have gone off, there's no wake. You know, did he kill it? And he sees an oil slick. You know, that a very good indication that he hit it. In his report, Claudius was clear.

HERBERT CLAUDIUS (dramatized)

It is my opinion that the sub was sunk or so mortally wounded that she would never return to her base.

BALLARD

I mean, this is the fog of war. But he is pretty confident that that oil slick is associated with that submarine.

KOHLER

They attack the submarine. They come back and rescue the survivors, the 400 people that are in the water from the Robert E. Lee.

NARRATOR

Claudius radioed for help, knowing that his ship was too small to rescue everyone.

CLAUDIUS

It's not that big a ship. It was so top heavy that he actually had to unload some of them back into lifeboats so that his ship became stable. But then two other ships came out and between the three boats, they took them back to New Orleans.

NARRATOR

Claudius returned to port with the survivors. Then, a shock.

KOHLER

Instead of getting a hero's welcome, he was actually reprimanded. His entire attack is criticized. As a matter of fact, he's removed from command and then sent back to school. They didn't believe, not for one minute, that he had actually sunk the U-166.

NARRATOR

Senior commanders concluded that Claudius made a series of basic errors. They said he was in the wrong position while escorting the Robert E. Lee; he approached the U-boat the wrong way; and crucially, he deployed his depth charges too slowly and at the wrong depths.

BALLARD

They're all saying, "No way. "No way did he sink this sub. "The attack was poorly conducted, "and there is insufficient evidence to give higher assessment than an F." An F! Says "F" right there. Flunked. That's pretty humiliating.

NARRATOR

Gordon Claudius, Herbert's son, was only two years old at the time. He believes the review of his father was unfair.

CLAUDIUS

It was not favorable. He didn't talk much about... well, his wartime activities. My sister, she was older than I was and more in a position to think about things and ask questions, and she said, well, she asked him one time and all he said was, well, he attacked a submarine and he saw an oil slick and he saw debris. That was it. I think it not only got to my father, but I think it got to the whole crew.

NARRATOR

After the war, captured German records revealed that U-166 was the only U-boat lost in the Gulf. But the Navy concluded that U-166 had been sunk in an entirely different attack that took place two days later and 140 miles from where commander Claudius gave chase.

MORGAN

A U-boat is spotted south of Houma, Louisiana, running on the surface. (explosion) And a U.S. Coast Guard patrol plane attacks it with depth charges, and then in the aftermath of the attack observes an oil slick on the surface of the water.

NARRATOR

The Coast Guard air crew was given credit for the kill, and so official history was written. Yet despite decades of searching, a wrecked U-boat was never found at the location where the Coast Guard plane made its attack.

MORGAN

It's not for a lack of trying. People are going out on dive expeditions thinking that they have the exact spot where U-166 went down, but nobody finds it.

NARRATOR

Until 2001, when marine archaelogists from C&C Technologies made a surprising discovery during preparations for an undersea pipeline.

DANIEL WARREN

When we were looking at the extra data and we saw the bow section. And at that point we looked at each other and we were like...

WARREN

All the pieces came together. And it turns out it was the U-166.

CHURCH

We knew in that moment that Lt. Commander Claudius and the crew of PC-566 had sank U-166.

NARRATOR

Yet 13 years later, the Navy record still denies Claudius credit.

BALLARD

So now we need to set the record straight, because this guy died without recognition for what he did.

NARRATOR

Unless Bob Ballard and Richie Kohler can find a way to prove to the Navy that Claudius was responsible, the official record will stand as is. A mile below, Ballard's ROVs are closing in on the wrecked U-boat.

BALLARD

All right, showtime. Let's drop down there. There it is. Thar she blows.

NARRATOR

She is in incredible condition, right where the previous survey said she would be.

KOHLER

The aft part of the submarine looks like we could just blow off the dust, start the engines and go. (chuckling): Right.

NARRATOR

250 feet long, weighing 1,100 tons, in 1942, U-166 was a state-of-the-art killing machine. Now she is a tomb. Oh, my gosh. That's a gun. So far, no damage is apparent.

BALLARD

Back up a little. Frame it a little. Right there, okay.

NARRATOR

Though Richie Kohler sees evidence that the crew of U-166 knew they were in danger.

KOHLER

There is a couple of telltale signs that this sub was in a crash dive or trying to get down real quick. Number one, the aerial, the antenna that you see that's bent, that's a transmitting antenna.

BALLARD

So it wasn't making... It's supposed to be retracted when they're diving. So he was too busy to do that? Everything stopped. We've got the 20 millimeter gun that should have been locked in position for underwater travel. It's swung out to port. The periscope never came back down all the way. You can almost, you know, see these men are running to the forward end of the submarine trying to get the bow heavier, trying to get it down, because they knew trouble was coming. And they didn't make it. They didn't make it.

NARRATOR

It does appear the sub was running for its life. But so far, they see no sign of the tell-tale fractures in the hull that two rounds of depth charges should have produced. The wreck seems surprisingly intact. And then, something strange.

BALLARD

Now, that's not normal. Could you stop right there, Will, and zoom in on that?

NARRATOR

Ballard zooms in, looking for the bow, the front of the sub. It appears to be buried in the sand, but it's not. It's completely gone. There's no way a depth charge could have sheared off the bow of U-166 like this. So what happened? The missing bow holds vital clues.

BALLARD

We gotta go find the other piece.

NARRATOR

The ROVs move out across the sand. For many meters, there's nothing. Then suddenly... It's the missing bow. And it's been reduced to scrap metal. This is definitely not a depth charge.

KOHLER

If that was a depth charge, we would not see this thin lattice work. This would've been totally blown away.

NARRATOR

Normally, a depth charge just cracks the pressure hull. But this is no fracture; it's an amputation.

KOHLER

Most of the time, we see concave indents from depth charges. We don't see twisted and torn metal. With this level of destruction, it makes it difficult to ascertain what caused what.

NARRATOR

It's a conundrum. The location supports Claudius's claim that he sank this boat, but the damage doesn't match a normal depth charge attack like the one Claudius made. To understand what happened, the team needs to put the two pieces of U-166 back together. We know that we've got this up in the bow, separated, and then we don't know how much...

BALLARD

Well, you know, we take the two pieces and we see how much of it we see. Put them together and you see what you're missing.

NARRATOR

But the visibility is too murky to image the entire wreck in one shot. We can't see very far underwater. I mean, if you're lucky you can see 30, 40 feet. Fortunately, Ballard's ROVs are equipped for just such conditions.

BALLARD

What we'll do now is we've now outlined, and so we need to bring Clara up. Is Clara in the ready? So we can digitize this whole thing.

NARRATOR

Clara Smart is the team's high resolution mapping specialist. As she watches, the ROV does a sweep of the wreck, taking thousands of close-up photographs with its ultra-high resolution cameras. The vehicle takes one image every three seconds, and what we're going to do is we're going to combine all these images to create...

BALLARD

One.

SMART

One very big, beautiful image.

BALLARD

You can see it's marching along. Yep, and it's just matching one picture to the next. The whole idea overall is when we are on a site, we have a flashlight in a hay field and you can't see anything, but once we make these maps, all of a sudden, like all the lights came on so we truly see exactly what's down there.

NARRATOR

Clara will spend the next several months stitching the thousands of close-ups into a single giant wide shot, called a photo mosaic, that shows the entire wreck. Every rivet and crack will be visible with unmatched clarity.

BALLARD

Here, we're able to get a picture you can't get any other way, a map that shows you what it would look like if you could take the water away. And that'll help tell us what happened to the submarine.

NARRATOR

Back at the University of Rhode Island, Clara gets to work. To start, computers assemble the images. But they can only do so much.

SMART

As you can see, we've got some issues.

NARRATOR

In problem areas, Clara will have to match individual frames by eye. It's a laborious process. Finally, Ballard calls Richie Kohler and historian Marty Morgan to his lab. The mosaic of U-166 is complete. Thousands of photographs have been seamlessly meshed together. But will the mosaic give the team what they need?

SMART

We took about 2,000 images with the ROV, and that's been boiled down to these two mosaics.

KOHLER

So this is really CSI. We're talking about 2,000 pictures to give us one continuous image of what the U-166 looks like now. Right, and it would be as if you were flying over it with an airplane and you saw an aerial view.

NARRATOR

The key evidence lies somewhere in the break between the two pieces.

BALLARD

That's where it all took place. The fact that this has been completely blown off...

SMART

And 100 meters away.

BALLARD

And 100 meters away. You know, the question is, was the depth charge enough power to literally tear off the front of a pressure hull? It's not really been seen before. In almost every instance... So you're led to believe that there was another culprit in the mix.

NARRATOR

When the two pieces of U-166 are slid back together, they see that the break occurred right at the forward torpedo room.

KOHLER

Interesting enough, right here, where my finger is, is exactly where the torpedo tubes would have been loaded. You can see them clearly in the blueprints. We know they're there. But something destroyed the torpedo tubes.

BALLARD

And there's how many torpedoes here? There could have been four. Four on the deck, four spare reloads.

NARRATOR

Spare torpedoes were stored on the floor of the two torpedo rooms, bow and stern. If the forward torpedoes somehow exploded while stored inside U-166, that could explain the incredible damage to her bow. If that is what happened, it points to an unlikely and catastrophic chain of events. As U-166 frantically dove to escape, Commander Herbert Claudius dropped his depth charges.

BALLARD

So they were pretty shallow. Well, they were seen on the surface with their periscope up.

MORGAN

This is what has led me to believe the possibility of one of the five depth charges that were distributed by PC-566 landed on the deck there.

BALLARD

And it carried it with 'em.

NARRATOR

Marty Morgan believes a depth charge landed directly on top of U-166. As the sub dove to escape, she carried the bomb down to its explosion depth, setting off a chain reaction that detonated her own torpedoes. That would make sense because clearly it was so instant, he's just putting his periscope down. His dive angle isn't that great... He probably wasn't even 30 feet deep. Right, so it very conceivably landed on him, clunk, and he carried the bomb. Carries it down with him. Hustling forward to weigh down the bow, the crew might have run right into the exploding torpedo room.

KOHLER

If you understand how the German submarines would dive, one of the things they would use the crew for is ballast. They would tell the crew to run. "Everyone run to the front!" And that's exactly where it happened.

NARRATOR

The team is convinced. While a depth charge couldn't produce the damage seen on U-166, it could have detonated her torpedoes. Combined with the U-boat's location, it makes a persuasive argument. Instead of being reprimanded for his attack, Commander Herbert Claudius should have gotten a medal. Nice shot. Perfect shot, actually. Couldn't have done it better.

But the team still faces a huge hurdle

convincing the U.S. Navy. Ballard, a former Navy commander himself, puts their findings in writing and forwards them to the Chief of Naval Operations, the most senior officer in the Navy. 73 years after the battle, the Navy agrees to review the case. Though it wasn't apparent in 1942, even as U-166 sank to the bottom, Operation Drumbeat was already drawing to a close.

BALLARD

The sinking of 166 This is the first time we actually, in the Gulf of Mexico, drew blood.

KOHLER

Didn't mean that we had taken the teeth away from the U-boats. No, they were going to continue to sink ships, but now it was going to cost them, it was going to cost them dearly.

NARRATOR

Allied science and engineering were finally beginning to turn the tide. Improved radar and high-frequency direction-finding meant the U-boats could be detected whenever they surfaced, even at night, while mass production of aircraft like the B-24 Liberator meant the Atlantic Gap was no longer a safe haven for the Milk Cows. But the decisive stroke came with the capture of U-559 in October of 1942.

MULLIGAN

What they get out of U-559 enables Allied codebreakers to regain that insight into the new Enigma machine with its fourth rotor. And by the end of December 1942 and certainly by the spring of 1943, the British and Americans can now read German U-boat signals in almost real time.

NARRATOR

The Enigma code was cracked once again. The U-boats had lost nearly all their advantages. Even the Nazi spies of Operation Pastorius, landed via U-boat to sabotage American industry, proved to be utter failures. All eight operatives were captured within days, and the Germans canceled the program. Never again would U-boats rule the seas.

They had failed in their stated goal

cutting off the flow of supplies from America to England. Yet they had come dangerously close, and the damage caused by their attacks was immense. Hitler's U-boats sank 609 ships in American-protected waters. Over three million tons of cargo never made it to Britain, and over 5,000 lives were lost. On the German side, out of the 743 U-boats lost in World War II, only ten were sunk in American waters. Of those,

only one was sunk in the Gulf of Mexico

U-166. But will the Navy give credit to Commander Herbert Gordon Claudius? At the Navy History and Heritage Command, historians have analyzed Ballard's evidence, as well as reports from the marine archaelogists that first IDed U-166. The two teams are nearly lockstep in their conclusions. It was Commander Claudius and his naval escort ship PC-566.

ROBERT NEYLAND

The Underwater Archaeology Branch here at Naval History and Heritage Command looked at the information and confirmed that absolutely, we believe that PC-566 did successfully attack and sink U-166. Whether it was skill, whether it was luck or a combination of both, they were successful in the end.

NARRATOR

In 2014, 72 years after the battle, Admiral Jonathan Greenert, chief of naval operations, and Secretary of the Navy Ray Mabus award Commander Herbert Claudius the Legion of Merit.

ADMIRAL GREENERT

Good afternoon, everybody, and we're here to recognize and actually to honor Lieutenant Commander Herbert G. Claudius. This is really for me a story, I think, of history obviously, but also a story of explorers, of shipmates, of friends, of historians, and I think relentlessness to set the record straight.

NARRATOR

Gordon Claudius, the only surviving child of Herbert Claudius, is here to accept the award on his father's behalf. Now, 70 years later, because of technology, we now know that your father's after-action report was absolutely accurate. And I think this is a good example of, "It's never too late to set the record straight, it's never too late to do the right thing." So it's an honor to be here today to present your father posthumously with the Legion of Merit for valiant actions during a very tough and very dangerous combat situation. On behalf of your father. Thank you. I present this to you with the V for Valor, which means it happened in combat. It sure did. (applause)

BALLARD

This really brings closure on a story that began a month before I was born 73 years ago. So this is a wrap, a nice wrap.

NARRATOR

So long after the conflict, World War II is fading history. Few remember the battles Herbert Claudius and other heroes once fought so close to our shores, but the sunken remains are still there, often nearer than we know, enduring reminders of just how close the Nazis came to setting history on a very different path. In the earth's oceans, a deadly recipe is brewing.e, where you can watch this and other NOVA programs. See expert interviews, interactives, video extras, and more. Follow NOVA on Facebook and Twitter and find us online at pbs.org/nova. This NOVA program is available on DVD. To order, visit shopPBS.org, or call 1-800-PLAY-PBS. NOVA is also available for download on iTunes.