This is a maar, a lake that forms within the crater of an extinct volcano.
This one, Lac de Servières, is an 8000 year old volcano. The lake was formed from a phreato-magmatic eruption, which is an explosive type of eruption where water and magma are mixed.

This is a maar, a lake that forms within the crater of an extinct volcano.

This one, Lac de Servières, is an 8000 year old volcano. The lake was formed from a phreato-magmatic eruption, which is an explosive type of eruption where water and magma are mixed.