Ijen Volcano / Indonesia, East Java
The Ijen volcano complex is a group of stratovolcanoes inside a larger caldera Ijen, which is about 20 kilometers wide. The Gunung Merapi stratovolcano 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 (en.wikipedia.org/wiki/Ijen).