It would explode with a force a thousand times more powerful than the Mount St Helens eruption in 1980.
Spewing lava far into the sky, a cloud of plant-killing ash would fan out and dump a layer 10ft deep up to 1,000 miles away.
Two-thirds of the U.S. could become uninhabitable as toxic air sweeps through it, grounding thousands of flights and forcing millions to leave their homes.
This is the nightmare that scientists are predicting could happen if the world’s largest super-volcano erupts for the first time in 600,000 years, as it could do in the near future.
Yellowstone National Park’s caldera has erupted three times in the last 2.1million years and researchers monitoring it say we could be in for another eruption.
They said that the super-volcano underneath the Wyoming park has been rising at a record rate since 2004 – its floor has gone up three inches per year for the last three years alone, the fastest rate since records began in 1923.
But hampered by a lack of data they have stopped short of an all-out warning and they are unable to put a date on when the next disaster might take place.
When the eruption finally happens it will dwarf the effect of Iceland’s Eyjafjallajökull volcano, which erupted in April last year, causing travel chaos around the world.