VEI 8

A supervolcano is any volcanic system that can produce an eruption rated VEI 8 on the Volcanic Explosivity Index — throwing out more than 1,000 cubic kilometers of rock and ash.

To feel that scale: the 1980 Mount St. Helens blast, which flattened forests for miles, ejected about 1 cubic kilometer. A supereruption is at least a thousand of those at once.

When a supereruption empties the magma chamber, it drains so fast that the ground above loses its support and collapses inward. The result is a giant bowl-shaped sink called a caldera — not a cone-shaped mountain like most people picture.

Yellowstone's caldera is so wide it is hard to see from the ground: roughly 72 kilometers long and 48 wide (about 45 by 30 miles). Visitors stand inside it without realizing it.