A caldera is the low rim left behind when a large volcano collapses in upon itself after a series of very large eruptions. (more below)

This caldera is Ugashik, the ring of ridges around the five domes of Peulik volcano, on the Alaska Peninsula. Calderas tend to form after a series of large eruptions. Usually a new volcano starts to grow inside, eventually covering it. This is a typical caldera, less well-formed than the Crater Lake caldera.

Caldera-forming eruptions are the largest on Earth. The conditions in the heart of one of these explosions are beyond hellish.

Yet geologists are beginning to model them.