Swarms of small earthquakes happen frequently in Yellowstone, but it's very unusual for so many earthquakes to happen over several days, said Robert Smith, a professor of geophysics at the University of Utah.
"They're certainly not normal," Smith said. "We haven't had earthquakes in this energy or extent in many years."
It is little known that lying underneath one of America's areas of outstanding natural beauty - Yellowstone Park - is one of the largest supervolcanoes in the world. Scientists have revealed that it has been on a regular eruption cycle of 600,000 years. The last eruption was 640,000 years ago... so the next is overdue.
And the sleeping giant is breathing: volcanologists have been tracking the movement of magma under the park and have calculated that in parts of Yellowstone the ground has risen over seventy centimetres this century. Is this just the harmless movement of lava, flowing from one part of the reservoir to another? Or does it presage something much more sinister, a pressurised build-up of molten lava?
Scientists have very few answers, but they do know that the impact of a Yellowstone eruption is terrifying to comprehend. Huge areas of the USA would be destroyed, the US economy would probably collapse, and thousands might die.
And it would devastate the planet.
The last supervolcano to erupt was Toba 74,000 years ago in Sumatra. Ten thousand times bigger than Mt St Helens, it created a global catastrophe dramatically affecting life on Earth. Scientists know that another one is due - they just don't know when... or where. Climatologists now know that Toba blasted so much ash and sulphur dioxide into the stratosphere that it blocked out the sun, causing the Earth's temperature to plummet. Some geneticists now believe that this had a catastrophic effect on human life, possibly reducing the population on Earth to just a few thousand people.
Mankind was pushed to the edge of extinction... and it could happen again.
