The supervolcano quietly bubbling away beneath Yellowstone National Park could devastate the western half of the USA if it erupts.
Fears are growing a huge eruption is due, 630,000 years after the last blast sent debris flying hundreds of miles across America.
A modern eruption of the Yellowstone volcano, however, would explode over a very different America, with tens of millions of people now living on the land once covered with lava and ash.
These US cities are at risk in the event of another huge Yellowstone volcanic eruption.
Denver City is the biggest city at direct risk of a major blast, located around 500 miles to the south-east of the volcano.
In millennia past, the area had been covered by up to three feet of ash, which moved at “hurricane speeds” and boasted terrifyingly-hot temperatures as it flew across the country.
Doctor Hank Heasler, park geologist at Yellowstone, said: “This ash when it comes up is maybe about 2,000 degrees Fahrenheit.
“So very hot and then it moves laterally at hurricane force winds and that’s what causes a wide zone of destruction.”
Denver is home 600,000 people with as many as two million others living in the metro area surrounding Colorado’s capital.
Salt Lake City, located one state west in Utah, is another major conurbation at risk form a Yellowstone volcanic eruption.
Here, like Colorado, as much as three feet of ash could fall, smothering the city and blotting out the sun.
More than 180,000 people live in the city with more than one million inhabiting the surround metro area.
The damage would increase exponentially in the smaller towns and cities nearer Yellowstone volcano itself.
Billings, Montana would see several feet of ash fall, potentially piling above head-height of the city’s 100,000 inhabitants.
Boise, Idaho and Rapid City, South Dakota would likewise be swamped while an even worse fate would await the smaller towns and villages within Yellowstone itself or less than 100 miles from the volcano.
People living in these areas would see “tens of feet” of ash fall, covering whole towns in a sea of ash and debris.
Doctor Harley Benz of the US Geological Survey said huge eruptions in the past had left nothing in its wake.
He said: “We’re talking about a huge area that was covered in tens of feet of ash, in a very large area out to 100 miles from the centre.”