It’s the awe-inspiring pride of the United States – and it harbors a deadly power that could kill us all.

Yellowstone National Park is pristine wilderness, full of scenic landscape and iconic hot-pools and geysers that attract tens of thousands of visitors every year.

But it’s what lies beneath that has scientists scurrying.

We’ve long known that Yellowstone is merely the skin on top of a supervolcano – a giant pool of magma sitting just under the Earth’s surface.

Exactly how giant has been the subject of much speculation.

Until now.

A team from the University of Utah have told the American Geophysical Union Fall Meeting in San Francisco that Yellowstone’s magama chamber is 2.5 times larger than previously thought.

It is an underground cavern that measures some 55 miles by 20 miles and runs between 3 and 9 miles below the earth.

If it blows it will wipe out America – and have enormous impacts on the rest of the world.

The university researchers described their discovery as “astounding.”

Professor Bob Smith told the BBC: “We’ve been working there for a long time, and we’ve always thought it would be bigger … but this finding is astounding.”

The research is part of an ongoing effort to assess the true threat the molten beast represents.

What is a supervolcano?

The common picture of a volcano being a mountainous cone of ash and lava does not apply to the supervolcanos like Yellowstone.

These are vast spaces of collapsed crust that formed pools – known as calderas – under a seemingly normal surface. Only mapping reveals the gentle swell, over a space of hundreds of square kilometres, that contains the cauldron of molten magma below.

Eruption ‘due’

From analysis of rock and sediment layers, scientists say another eruption is almost due – at least by geological standards.

It appears the supervolcano explodes roughly once every 700,000 years.

Three such eruptions are known: One was 2.1 million years ago. Another was 1.3 million years ago.

The most recent was 640,0000 years ago.

Big Bang

So what would happen if Yellowstone was to erupt?

Something close to Armageddon.

Soil samples reveal that the last time it happened the whole of North America was smothered by ash. The lava flow was almost as great.

The streams of molten rock were hundreds of miles long, and miles thick.

Such was the extent of the smoke and debris cloud generated by the eruption that the climate of the entire world was affected for several centuries.

Measuring the beast

The ongoing rumbles caused by earthquakes in and around Yellowstone National Park provided the means by which the full extent of the magma chamber was revealed.

As the seismic waves moved through the ground, the different speeds of their travel was recorded by a network of seismometers.

“The waves travel slower through hot and partially molten material … with this, we can measure what’s beneath.” Dr. Jamie Farrell, from the University of Utah, said.

But wait: There’s more

Twenty “smaller” supervolcanoes have been found nearby, on the Utah/Nevada state border.

The new study published in the journal Geosphere shows that these volcanoes are not active today. But, 30 million years ago, they spilt more than 5500 cubic kilometres of magma during a one-week period.

“In southern Utah, deposits from this single eruption are 4km thick. Imagine the devastation – it would have been catastrophic to anything living within hundreds of miles,” Dr. Eric Christiansen of Brigham Young University said.

This article originally appeared on News.com.au.