Caldera
A huge basin left when a volcano empties its magma chamber in a great eruption and the ground above collapses into the space.
A caldera is a large, basin-shaped volcanic hollow, many times wider than the vents that fed it, typically from 2 to 50 kilometres across. It is not just a big crater. Craters are mostly blasted out by explosions; calderas form by collapse.
Beneath many volcanoes lies a shallow reservoir of magma. In a very large eruption so much magma can pour out, so quickly, that the rock above the reservoir loses its support. The summit then sinks into the emptied space, leaving a wide depression with steep walls. Over time the basin may fill with water.
Crater Lake in Oregon is a famous example. About 7,700 years ago, the volcano Mount Mazama had a climactic eruption and its top collapsed; rain and snow later filled the caldera to make the deepest lake in the United States. The eruption spread ash across much of north-western North America, and things buried beneath that ash, including some of the oldest known shoes, can be dated by it.
Calderas are not necessarily dead. Some, such as Yellowstone, still have magma below and show it through hot springs, geysers and the slow rising and sinking of the ground. New eruptions can also build fresh cones on a caldera's floor, as Wizard Island in Crater Lake shows.