The Ijen volcano complex is a group of composite volcanoes located on the border between Banyuwangi Regency and Bondowoso Regency, East Java, Indonesia. It is known for its blue fires, acid crater lakes, and labor-intensive sulfur mining.
It is within the larger Ijen caldera, which is about 20 kilometers (12 miles) wide. The Mount Merapi stratovolcano is the highest point of the complex. The name “Mount Merapi” means ‘volcano’ in Indonesian; Mount Merapi in Central Java and Marapi in Sumatra have the same etymology.
To the west of Mount Merapi is the Ijen volcano, which has an acidic, turquoise crater lake one kilometer (0.62 mi) wide. The lake is the site of labor-intensive sulfur mining operations, where baskets containing sulfur are carried by hand from the bottom of the crater. The work is well paid considering the cost of living in the area, but it is very strenuous. Workers earn around US$13 per day and, once out of the crater, still need to carry their sulfur cargo about three kilometers to the nearby Paltuding Valley to get paid.
Many cones and other post-caldera craters are located within the caldera or along its rim. The greatest concentration of post-caldera cones stretches from east-west across the southern flank of the caldera. The active crater at Ijen Crater has a diameter of 722 meters (2,369 ft) and a surface area of 0.41 square kilometers (0.16 sq mi). It is 200 meters (660 ft) deep and has a volume of 36 cubic hectometers (29,000 acre⋅ft).
This lake is recognized as the largest highly acidic crater lake in the world. It is also a source for the Banyupahit river, producing highly acidic and metal-rich river water that has significant detrimental effects on the river’s downstream ecosystem. During a scientific expedition in 2001, the pH of the lake was measured at <0.3.  On 14–15 July 2008, explorer George Kourounis took a small inflatable boat into the acid lake to measure its acidity. The pH of the water at the edge of the lake was measured to be 0.5 and in the middle of the lake 0.13 due to the high concentration of sulfuric acid.