Kawah Ijen volcano (2386m) is an active stratovolcano located on the eastern part of Java Island. The top of Ijen volcano is characterized by a very acidic crater lake containing 27.5 million m³ of turquoise-colored water. This lake is the biggest reservoir of hot and acidic water on Earth.

On the southern part of the lake, fumaroles are ejecting tons of sulfur every day. At night, this site is one of the only spot on Earth where it is possible to see blue fires. This is due to very hot gas coming from the volcanic conduits burning the sulfur, which leads to an extremely beautiful scenery. On the western part of the crater, a dam was build hundred years ago upstream of the Banyu Pahit river.