Hike the Kawah Ijen on East Java 

The Kawah Ijen volcano complex is a group of stratovolcanoes which are located at Banyuwangi Regency on East Java. The caldera of Ijen, which is about 20 kilometers wide. The Gunung Merapi stratovolcano is the highest point of that complex. The name "Merapi" means "mountain of fire" 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 of sulfur mining operation, in which sulfur-laden baskets are carried by hand from the crater floor to the nearby Paltuding Valley to get paid.  The work is low-paid and very onerous… Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of post-caldera cones run east-west across the southern side of the caldera. The active crater at Kawah Ijen has a diameter of 722 meters and a surface area of 0.41 square kilometers (0.16 sq mi). It is 200 meters deep and has a volume of 36 cubic hectometers . The lake is recognized as the largest highly acidic Crater Lake in the world. It is also a source for the river Banyupahit, resulting in highly acidic and metal-enriched river water which has a significant detrimental effect on the downstream river ecosystem.