Yellowstone National Park is renowned for its scenic geysers and hot springs covering some 2,300 square miles (3,800 square kilometers). The energy for all this hot water actually comes from volcanic energy below a plateau that is actually a giant caldera. Geologists estimate that this massive volcano last erupted about 640,000 years ago with an energy equal to 8,000 Mount St. Helens eruptions. One can only imagine the destruction such an explosion would have wreaked. A nuclear winter would have ensued, acid rain would have fallen from the sky, and some scientists believe that the human race was driven almost to extinction at the time.

Since the volcano is still active, it could happen again at almost any time. Some parts of Yellowstone have risen about 29 inches (74 centimeters) since measurements were first taken in 1923, which indicates a buildup of magma beneath the crust. It is only a matter of time before this energy is released. And Yellowstone is not the only such supervolcano. The last such eruption came from Toba, Sumatra, about 70,000 to 75,000 years ago, and other, as-yet-undiscovered supervolcanoes might exist.