There’s a powerful and violent menace lurking under northwestern Wyoming and southeastern Montana, one that has reshaped the landscape several times over the last several million years. It’s called the Yellowstone Supervolcano and the resulting geysers, bubbling mudpots, hot springs, and evidence of long-gone volcanoes make Yellowstone National Park a fascinating geologic wonderland. The official name for this region is the “Yellowstone Caldera”, and it spans an area about 72 by 55 kilometers (35 to 44 miles) in the Rocky Mountains. The caldera has been geologically active for 2.1 million years, periodically sending lava and clouds of gas and dust into the atmosphere, and reshaping the landscape for hundreds of kilometers. Yellowstone Caldera is among the world’s largest such calderas. The caldera, its supervolcano, and the underlying magma chamber help geologists understand volcanism and is a prime place to study first-hand the effects of hot-spot geology on the Earth’s surface. The Yellowstone Caldera is really the “vent” for a large plume of hot material that extends hundreds of kilometers down through Earth’s crust. The plume has persisted for at least 18 million years and is a region where molten rock from Earth’s mantle rises to the surface. The plume has remained relatively stable while the North American continent has passed over it. Geologists track a series of calderas created by the plume. These calderas run from the east to northeast and follow the motion of the plate moves to the southwest. Yellowstone Park lies right in the middle of the modern caldera. The caldera experienced “super-eruptions” 2.1 and 1.3 million years ago, and then again about 630,000 years ago. Super-eruptions are massive ones, spreading clouds of ash and rock over thousands of square kilometers of the landscape. Compared to those, smaller eruptions and the hot-spot activity Yellowstone exhibits today are relatively minor. The plume that feeds the Yellowstone Caldera moves through a magma chamber some 80 kilometers (47 miles) long and 20 km (12 miles) wide. It is filled with molten rock that, for the moment, lies fairly quietly below Earth’s surface, although from time to time, the movement of the lava inside the chamber triggers earthquakes. Heat from the plume creates the geysers (which shoot superheated water into the air from underground), hot springs, and mudpots scattered throughout the region. Heat and pressure from the magma chamber is slowly increasing the height of the Yellowstone Plateau, which has been rising more rapidly in recent times. So far, however, there is no indication that a volcanic eruption is about to occur. Of more concern to scientists studying the region is the danger of hydrothermal explosions in between major super-eruptions. These are outbursts caused when underground systems of superheated water are disturbed by earthquakes. Even earthquakes at a great distance can affect the magma chamber.