A crater lake is a lake that fills the top of a volcano. It occupies the space of a former crater.

How a crater lake forms.

Crater lakes form when a volcano explodes so violently that it leaves the magma chamber below it empty. At the same time, the enormous explosion causes many earthquakes in the rocks above, and they begin to crack and weaken.

The top of the volcano then collapses down into the magma chamber, leaving a huge circular pit known as a caldera.

In time, the caldera fills with water to make a crater lake.

This is Crater Lake, Oregon, United States.

This is Crater Lake, Oregon, United States, from the air.

A new cone may grow again, as is the case for Crater Lake, Oregon. The small island - Wizard Island - that you see is actually a cone 700m high in a lake 600m deep.