Crater lake
This page describes the geological feature. For the lake of the same name, see Crater Lake.
A crater lake is a lake that forms in volcanic calderas or craters after the volcano has been inactive for some time. The best-known crater lake, which bears the same name as the geological feature, is Crater Lake in Oregon state's Mount Mazama. Some crater lakes are acidic, however, some lakes' water remains fresh. Some also have geothermal activity, especially if the volcano is merely dormant rather than extinct.
Some of these lakes manifest in craters formed by impact events. Lacking ongoing volcanic processes, these do not tend to develop acidity or geothermal activity.