NOVA

- [Narrator] Tonga is still recovering from the eruption of the Hunga volcano.

(ominous music) Now scientists have discovered that its volcanic twin, Tofua, is far more active than satellites have led them to believe, a finding that has implications far beyond Tonga.

- [Shane] We had a big eruption around 1,200 years ago, another big one around 800 years ago, and the most recent one around 400 years ago.

(ominous music) - [Narrator] This pattern suggests Tofua should erupt again, and soon.

(ominous music) - [Shane] Okay, this is fantastic.

We've got a bit of a break in the weather.

You can see how blue that gas is coming off that volcano.

(ominous music) When the color of the gas is blue, it's sulfur dioxide.

(ominous music) - [Narrator] This gas is released by the magma.

(ominous music) The blue color is only visible if the levels of sulfur dioxide are high.

- I'm going to try and get a position close to the plume.

I don't want to go underneath it, because sulfur dioxide us actually quite lethal.

We can then try to measure some of the output.

This is what I've been lugging up the hill.

(bag rustling) - [Narrator] Volcanologists monitor the sulfur dioxide to gauge the activity of volcanoes.

- [Shane] This is a telescope.

and it's a rotating one.

- [Narrator] The more sulfur dioxide Shane detects, the more active the volcano.

- Now, it's gonna be scanning over there, and I want it to scan above the level of the opposite caldera rim.

We're capturing it beautifully with this angle.

- [Narrator] Shane's readings indicate the satellites have been dangerously underestimating the levels of sulfur dioxide.

Tofua is releasing far more than previously thought.

Toniella's expert flying reveals that the magma has welled up in the crater, creating a lava lake, one of fewer than 10 on the entire planet, and there's a sign Tofua has been actively throwing out lava incredibly recently.

- This erupted in the last few days.

It's a fresh volcanic bomb, so this really contrasts to all of the rest of the older material around here.

So there's been some quite big explosions to produce these.

It's really interesting, because it looks as if Tofua's entering a new phase of activity, one that we haven't seen here before.

Everything that we've discovered today is absolutely new, and I think will be of quite a shock around the globe.