Volcanic calderas

In volcanology the term caldera means a large circular or elliptical basin or depression, often occupied by a lake.
Meccanismi di formazione di una caldera vulcanica
The caldera is normally formed after a very violent eruption in which the magma chamber of the volcano is emptied and its roof collapses due to the void created: this collapse on the surface creates a sinking with the same shape as the magmatic chamber.
In subsequent eras, when the volcano is active again, the new volcanic structure is very often formed inside the caldera.
Often, due to their concave shape, lakes are formed inside the calderas due to the accumulation of rainwater.