It is inside a larger caldera Ijen, which is about 20kilometres wide. The Gunung Merapi stratovolcano is the highest point of that complex. The name "Gunung Merapi" means "mountain of fire" in the Indonesian language (api being "fire"); Mount Merapi in central Java and Marapi in Sumatra have the same etymology.
West of Gunung Merapi is the Ijen volcano, which has a one-kilometre-wide 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 Rp 50,000 - 75,000 ($5.50-$8.30) 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 722 metres (2,369ft) and a surface area of 0.41 square kilometres (0.16sqmi). It is 200 metres (660ft) deep and has a volume of 36 cubic hectometres (29,000acreft).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. In 2008, explorer George Kourounis took a small rubber boat out onto the acid lake to measure its acidity. The pH of the water in the crater was measured to be 0.5 due to sulfuric acid.
Since National Geographic mentioned the electric-blue flame of Ijen, tourist numbers increased. The phenomenon has occurred for a long time, but beforehand there was no midnight hiking. 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 sulphuric gas, which emerges from cracks at temperatures up to 600 degrees Celsius (1,112 degrees Fahrenheit).
The flames can be up to 5 metres (16 feet) high; some of the gas condenses to liquid and is still ignited. It is the largest blue flame area in the world and local people refer to it as 'Blue Fire'.
An active vent at the edge of the lake is a source of elemental sulfur, and supports a mining operation. Escaping volcanic gases are channelled through a network of ceramic pipes, resulting in condensation of molten sulfur. The sulphur, which is deep red in colour when molten, pours slowly from the ends of these pipes and pools on the ground, turning bright yellow as it cools. The miners break the cooled material into large pieces and carry it away in baskets. Miners carry loads ranging from 75 kilograms (165lb) to 90 kilograms (200lb), up 300 metres (980ft) to the crater rim, with a gradient of 45 to 60 degrees and then 3 kilometres (1.86 miles) down the mountain for weighing. Most miners make this journey twice a day. A nearby sugar refinery pays the miners by the weight of sulfur transported; as of September 2010, the typical daily earnings were equivalent to approximately $13 US. The miners often receive insufficient protection while working around the volcano  and complain of numerous respiratory afflictions. There are 200 miners, who extract 14 tons per day - about 20 percent of the continuous daily deposit.
Ijen and its sulfur mining was featured in the 1991 IMAX film Ring of Fire, and as a topic on the 5th episode of the BBC television documentary Human Planet. In the documentary film War Photographer, journalist James Nachtwey visits Ijen and struggles with noxious fumes while trying to photograph workers. Michael Glawogger film Workingman's Death is about sulfur workers.