Scientists monitoring a “supervolcano” underneath Yellowstone National Park in the United States have warned that it is may erupt for the first time in 600,000 years.

A swelling magma reservoir, which covers 925 square miles (an area the size of Los Angeles), has been rising at a record rate since 2004, according to scientists at the University of Utah. The floor has risen three inches per year for the last three years alone, the fastest rate since records began in 1923.

“It’s an extraordinary uplift, because it covers such a large area and the rates are so high,” said the University of Utah’s Bob Smith, an expert in Yellowstone’s volcanic activity.

He describes the probability of an eruption as “unlikely” since the reservoir is so far below the surface.

“At the beginning we were concerned it could be leading up to an eruption,” Smith added.

“But once we saw [the magma] was at a depth of ten kilometers, we weren’t so concerned. If it had been at depths of two or three kilometers [one or two miles], we’d have been a lot more concerned.”

However he predicts it will “eventually” happen, and when it does the aftermath is likely to be catastrophic.

The estimated eruption would be a thousand time more powerful than the Mount St Helens eruption of 1980. Ash would spew as high as 25 miles, plunging the earth into a “volcanic winter” that could last for years.

The toxic air would force millions to leave their homes, whilst the grounding of flights would dwarf the problems caused by the Iceland’s Eyjafjallajökull which blew its lid last April.

The National Park Service assures us: “There is no evidence that a catastrophic eruption at Yellowstone National Park (YNP) is imminent. Current geologic activity at Yellowstone has remained relatively constant since earth scientists first started monitoring some 30 years ago. Though another caldera-forming eruption is theoretically possible, it is very unlikely to occur in the next thousand or even 10,000 years. Scientists have also found no indication of an imminent smaller eruption of lava.”

Since the most recent blast 640,000 years ago there have been around 30 smaller eruptions, the most recent of which was 70,000 years ago.

They filled the caldera with ash and lava and created the flat landscape that draws thousands of tourists to Yellowstone National Park every year.

“Clearly some deep source of magma feeds Yellowstone, and since Yellowstone has erupted in the recent geological past, we know that there is magma at shallower depths too,” said Dan Dzurisin, a Yellowstone expert with the U.S. Geological Survey at Cascades Volcano Observatory in Washington State.

“There has to be magma in the crust, or we wouldn’t have all the hydrothermal activity that we have.

“There is so much heat coming out of Yellowstone right now that if it wasn’t being reheated by magma, the whole system would have gone stone cold since the time of the last eruption 70,000 years ago.”