Using specially commissioned research from the US Geological Survey, the Yellowstone Volcano Observatory and UK Met Office, part one of Supervolcano explores what would happen in the days leading up to such an eruption. Part two examines how humans would cope politically, environmentally, economically and socially in the aftermath of the volcano. On Supervolcano night, UKTV Documentary also brings you a 2-hour special about the making of the explosive 2-part series Supervolcano.

What's a supervolcano?
Imagine watching a volcano erupt. Deadly smoke and steam spew from beneath the earth's surface. Molten rivers of lava boil forth, melting the landscape and destroying everything they touch. Truckloads of ash and debris pelt down upon surrounding fields and cities. The unbearable, cataclysmic noise of the eruption is followed by the eerie silence of life destroyed. Now imagine 1000 volcanoes, all erupting at the same time, in the same place. That's a supervolcano.

When's the next one?
The last volcanic event of this magnitude happened about 74,000 years ago in Indonesia. It threw up so much ash that the world was plunged into darkness and a new ice age was triggered. From studying geologic records, scientist have discovered that supervolcanoes happen on Earth about every 50,000 years. So we're 24,000 years overdue and counting.

Why Yellowstone?
At Yellowstone National Park, the steamy, bubbling expanse of geothermal phenomena make it America's most popular tourist destination. What visitors see at the park is really just a light, crisp crust that hides a bulging pool of molten rock. On the earth's surface we see hot springs, geysers and bubbling mud pools. Underneath though, it's the world's biggest natural disaster waiting to happen.

During the past 2.1 million years, Yellowstone has erupted three times. Each super-eruption spewed up enough molten rock to make Mount Saint Helens look like a mere sneeze. Should Yellowstone erupt to this degree again, life on Earth as we know it would be destroyed forever.

How a volcano works
The Earth's crust is made up of vast pieces, called Tectonic plates, that slide around on the layer below - the mantle. Often, where the plates meet, one edge is forced down below the other, into the mantle. Water from the plate starts a reaction that causes the mantle to melt. The molten material is lighter than the crust above, so it forces its way to the surface, causing volcanic activity.

The situation at Yellowstone is slightly different. Yellowstone sits over a volcanic hotspot - a place in the mantle that is hotter and weaker than the surrounding areas. The extra heat melts the crust above, creating magma. The magma rises, forcing its way up to the surface, ultimately causing geysers, lava pools and volcanoes.