Ijen Volcano is located at the Eastern end of Java in Indonesia, close to Bali. Kawah Ijen is the world's largest highly acidic lake and is the site of a labor-intensive sulfur mining operation in which sulfur-laden baskets are hand-carried from the crater floor. Coffee plantations cover much of the Ijen caldera floor, and tourists are drawn to its waterfalls, hot springs, and dramatic volcanic scenery. The crater lake is beautiful.

The Ijen volcano complex is a group of stratovolcanoes inside a larger caldera Ijen, which is about 20 kilometers wide. The Gunung Merapi stratovolcano (not to be confused with Central Java's Gunung Merapi) is the highest point of that complex. 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. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of post-caldera cones forms an E-W-trending zone across the southern side of the caldera. The active crater at Kawah Ijen has an equivalent radius of 361 meters, a surface of 41 times 106 square meters. It is 200 meters deep and has a volume of 36 times 106 cubic meters.