What could cover the globe in ash, plunge Earth into an ice age and end life as we know it?
The answer is found in what lies beneath: supervolcanoes. Supervolcanoes are very rare. There is no need to run out and buy duct tape and plastic sheeting for this one. The last known supervolcano was about 74,000 years ago. But they are real, and one potential supervolcano lies right here in the United States, in one of America's most profound areas of natural beauty.
Just 20 miles beneath the earth's surface lies a pressurized ocean of molten rock looking for a way out. And a massive release of that molten rock would create a supervolcano -- arguably the largest natural disaster humanity would ever face.
Unlike regular volcanoes, which are shaped like mammoth cones, supervolcanoes spring from massive canyons -- calderas -- that measure hundreds of miles across. Underneath their surface is a vast lake of lava. When the underground liquid rock -- magma -- bursts forth to the surface, a series of violent, massive explosions could occur in a wide-ranging eruption that could last several days. It would incinerate anyone within a hundred miles, and layers of ash would blanket much of the earth.
"These eruptions are so big that you couldn't really see them, because you couldn't be close enough to the volcano, watching it and survive. You could watch it from a satellite and you could see the volcano erupt and see the ash cloud begin to spread," said Michael Rampino, geologist and professor of earth sciences at New York University.
The ash cloud would become so thick it could cover the sun, causing global temperatures to plummet.
Scientists say such an event wiped out almost the world's entire population 74,000 years ago, when a supervolcano erupted in Toba, near the Indonesian island of Sumatra. Only a few thousand people survived.
Supervolcanoes are little understood by scientists. Their underground canyons of molten rock are immensely vast, making their potential starting points hard to identify. It has been only in the last decade that scientists have started uncovering these deadly hot spots around the world, but they still don't know where they all are.
So far, scientists have identified nearly 40 possible supervolcano hot spots, including one right in our own backyard, underneath Yellowstone National Park. Scientists estimate that the Yellowstone area will experience a supervolcano eruption approximately once every 600,000 years. The last one occurred more than 630,000 years ago.
So how would we know a supervolcano is coming? And is there anything people could do to stop it or limit its destruction?
"We haven't seen a supervolcanic eruption, so we're not sure about what we will see," said John Grattan, a volcanologist at the Institute of Geography Earth Sciences at the University of Wales. "But one of the things that we would expect would be increased earthquake activity, an increase in the small geyser eruptions that you get in Yellowstone."
"The bottom line is that when one of these eruptions occurs, it's going to be a global disaster," said NYU's Rampino. "The only question is when and where."