Ijen volcano in East Java contains the world's largest acidic volcanic crater lake, called Kawah Ijen, famous for its turquoise color. The active crater measuring 950x600 m is known for its rich sulphur deposits which are being quarried.
The volcano is one of several active stratovolcanoes constructed over the 20 km wide Ijen caldera, the largest caldera in Java.
Eruptions from Ijen are very hazardous because of the risk of the lake draining to form catastrophic lahars.