The supervolcano hidden beneath Yellowstone national park in the US is even more enormous than previously thought, scientists have discovered.
A study found the volcano's magma chamber, which fuels the park's famous hot springs and geysers, is 2.5 times greater than had been estimated.
The underground cavern is now known to be more than 55 miles long and holds between 200 and 600 cubic km of molten rock. The findings have been hailed as "astounding" by scientists from the University of Utah who carried out the research.
Presenting their evidence at the American Geophysical Union Fall Meeting in San Francisco, the team played down the possibility of an increase in the threat of the dormant volcano, which last blew 640,000 years ago, following eruptions 1.3 million years ago and 2.1 million years ago.
The separations of roughly 700,000 years have led to speculation (thus far unfounded) that the next explosion could occur in 60,000 years.
The ash cloud generated from such a blast would have a range of 1,000 miles and leave two-thirds of the US uninhabitable.
But Professor Bob Smith said the new findings do not point to a far larger or more imminent catastrophe, telling the BBC: “Yes, it is a much larger system ... but I don’t think it makes the Yellowstone hazard greater.”
The team learned of the volcano's true vast size by placing a series of seismometers around the park.
Dr Jamie Farrell told the BBC: "We record earthquakes in and around Yellowstone, and we measure the seismic waves as they travel through the ground.
"The waves travel slower through hot and partially molten material ... with this, we can measure what's beneath."