Scientists monitoring it believe that a swelling magma reservoir six miles underground may be causing the recent uplifts.
They have also been keeping an eye on a ‘pancake-shaped blob’ of molten rock he size of Los Angeles which was pressed into the volcano some time ago.
But due the extreme conditions it has been hard to work out what exactly is going on down below, leading researchers unable to say with certainty what will happen - or when.
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 made 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.’
On the verge of a catastrophe? Yellowstone National Park's caldera has erupted three times in the last 2.1million years and scientists monitoring it say we could be in for another eruption
Scorched earth: An artist's interpretation of how the Midway Basin in the park might look after an eruption
July 22, 1980: Mount St Helens in Washington erupts. A Yellowstone caldera eruption would explode with a force a thousand times more powerful
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