The Ijen volcano complex is a group of stratovolcanoes in the Regency of Banyuwangi, East Java , Indonesia. It is inside a larger caldera Ijen , the which is about 20 kilometers wide . The Mount Merapi stratovolcano is the highest point of that complex . The name of this volcano resembles that of a different volcano , Mount Merapi in central Java , also known as Mount Merapi ; there is also a third named Marapi volcano in Sumatra . The name " Merapi " means " mountain of fire " in the Indonesian language .

West of Gunung Merapi is the Ijen volcano , the which has a one- kilometer - wide turquoise - colored acid crater lake . The lake is the site of a labor-intensive sulfur mining operation , in the 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 ( USD 50,000 - USD 75,000 ) per day and once out of the crater , still need to carry their loads of sulfur chunks about three kilometers to the nearby Pultuding Valley to get paid .

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 east / west - trending zone across the southern side of the caldera . The active crater at Kawah Ijen has an equivalent radius of 361 meters ( 1,184 ft ) , a surface square kilometers of 00:41 ( 0:16 sq mi ) . It is 200 meters ( 660 ft ) deep and has a volume of 36 cubic hectometres ( 29,000 acre · ft ) .

The lake is recognised as the highly acidic Reviews largest crater lake in the world , and since it is also a source for the river Banyupahit , resulting in highly acidic and metal - polluted water , it has a significant detrimental effect on the downstream river ecosystem . [ 3 ] In 2008 , explorer George Kourounis took a small rubber boat out onto the acid lake to measure its acidity . ThepH of the water in the crater was Measured to be 0.5 due to sulfuric acid