Kawah Ijen -  East Java - Indonesia

The Ijen volcano complex is a group of stratovolcanoes in the Banyuwangi Regency of East Java in Indonesia. It is inside a larger caldera Ijen, which is about 20 kilometers wide. The name of the highest volcano "Merapi" means „mountain of fire" in the Indonesian language. West of Gunung Merapi is the Ijen volcano, which has a one-kilometer-wide turquoise-colored acid crater lake. The lake is the site of a labor intensive sulfur mining operation, in which sulfur-laden baskets are carried by hand from the crater floor. The work is low-paid and very onerous. Workers earn around $5.50-$8.30 (Rp 50,000 - Rp 75,000) per day. The lake is recognised as the largest highly acidic crater lake in the world. In 2008 its acidity was measured to be 0.5 due to sulfuric acid.

An active vent at the edge of the lake is a source of elemental sulfur, and supports a mining operation. Escaping volcanic gasses are channeled through a network of ceramic pipes, resulting in condensation of molten sulfur. The sulfur, which is deep red in color when molten, pours slowly from the ends of these pipes and pools on the ground, turning bright yellow as it cools.

Sulfur can also self-ignite at temperatures as low as 248 'C and burns with a blue flame as sulfur dioxide gas is formed. These flames are a well-documented feature of Kawah Ijen and can be clearly seen at night.