A
crater lake is a
lake that forms in a
volcanic crater or
caldera, such as a
maar; less commonly and with lower association to the term a lake may form in an
impact crater caused by a meteorite, or in an artificial explosion caused by humans. Sometimes lakes which form inside calderas are called
caldera lakes, but often this distinction is not made. Crater lakes covering active (
fumarolic) volcanic vents are sometimes known as
volcanic lakes, and the water within them is often
acidic, saturated with
volcanic gases, and cloudy with a strong greenish color. For example, the crater lake of
Kawah Ijen in Indonesia has a
pH of under 0.5. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment.