NOVA

- [Narrator] When it comes to government coverups, perhaps the most infamous is Area 51.

Tales of alien spacecraft hidden inside this remote Air Force base about 83 miles northwest of Las Vegas have persisted for decades.

It turns out the military has been secretly developing cutting edge technology here since the 1950s, but did not even acknowledge the existence of the base until 2013.

- [Narrator 2] There are lots of aircraft we know have been tested at Area 51 entirely in secret at the time, including the F-117, the SR-71, the B-2 Spirit, and many more.

- Right around the same time the B-2 Spirit was in development, common sightings of UFOs around the United States tended to transition away from the pie saucer shape we were familiar with in earlier decades, and toward a more triangular shape.

This is a model of what people claim was the TR-3B, which is a legendary aircraft that people allege was built using reverse engineered alien technology.

- [Narrator] By the 1990s, rumors of triangular shaped spacecraft became so widespread they became part of popular culture.

Blockbusters like "The X-Files" featured a triangular-shaped alien spacecraft hovering over agent Fox Mulder's head.

(spacecraft whooshing) - I think the government should and does expect that the public is going to fill in the blanks as they see fit.

As long as people are talking about aliens operating out of Area 51, well, then they're not revealing anything about the legitimate military platforms that are being tested there.

It creates a bit of a smoke screen.

- There's always advanced technological programs going on that are on a need-to-know basis.

If you don't need to know, even if you are a member of the government or the military, you're not going to know.

And that's the way it should be.

- This is all about spy versus spy or military versus military to maintain technical superiority.

We don't want other people to learn what we're building, where we might employ it, because then they can defeat it.

There are reasons to do that.

None of it is alien.

None of it.

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