The Ijen volcano complex is a group of composite volcanoes located on the border between Banyuwangi Regency and Bondowoso Regency of East Java, Indonesia. It is known for its blue fire, acidic crater lake, and labour-intensive sulfur mining. It is inside an eponymous larger caldera Ijen, which is about wide. The Gunung Merapi stratovolcano is the highest point of that complex. The name "Gunung Merapi" means 'mountain of fire' in the Indonesian language; Mount Merapi in central Java and Marapi in Sumatra have the same etymology. West of Gunung Merapi is the Ijen volcano, which has a turquoise-coloured acidic crater lake. The lake is the site of a labour-intensive sulfur mining operation, in which sulfur-laden baskets are carried by hand from the crater floor. The work is paid well considering the cost of living in the area, but is very onerous. Workers earn around US$13 per day and, once out of the crater, still need to carry their loads of sulfur chunks about three kilometers to the nearby Paltuding 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 run east–west across the southern side of the caldera. The active crater at Kawah Ijen has a diameter of and a surface area of . It is deep and has a volume of . The lake is recognised 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. During a scientific expedition in 2001, the pH of the lake was measured at <0.3. On July 14–15, 2008, explorer George Kourounis took a small rubber boat out onto the acid lake to measure its acidity. The pH of the water at the lake's edges was measured to be 0.5 and in the middle of the lake 0.13 due to a high concentration of sulfuric acid. Blue fire crater Since National Geographic mentioned the electric-blue flame of Ijen, tourist numbers have increased. The phenomenon has long been known, but midnight hiking tours are a more recent offering. A two-hour hike is required to reach the rim of the crater, followed by a 45-minute hike down to the bank of the crater. The blue fire is ignited sulfuric gas, which emerges