Indonesia’s Kawah Ijen Volcano, on the island of Java, has two of the most unusual occurrences on Earth. The first is an active solfatara that emits hot, flammable sulfurous gases. These ignite as they enter Earth’s oxygen-rich atmosphere and burn with an electric blue flame. Some of the gas condenses in the atmosphere to produce flows of molten sulfur that also burn with an electric blue flame. The flames are difficult to see during the day but illuminate the landscape at night.
The volcano remains active. The last magmatic eruption occurred in 1817. Phreatic eruptions occurred in 1796, 1917, 1936, 1950, 1952, 1993, 1994, 1999, 2000, 2001, and 2002. These have caused very little damage but present a danger to anyone mining sulfur or visiting the caldera.
The second occurrence is a one-kilometer-wide caldera lake filled with turquoise-blue water. The color of the water is a result of its extreme acidity and a high concentration of dissolved metals. It is the world’s largest highly acidic lake with a measured pH as low as 0.5. 
The cause of its acidity is an inflow of hydrothermal waters charged with gases from a hot magma chamber below.
Electric blue flames caused by burning volcanic gases and molten sulfur. A night scene at the solfatara in the caldera of Kawah Ijen Volcano.
A continuous stream of sulfur-laden gases blasts from fumaroles at the lake-side solfatara. These hot gases travel underground in the absence of oxygen. If they are hot enough when they emerge from a vent, the sulfur ignites upon contact with oxygen in the atmosphere. Often the temperature is low enough that the sulfur condenses, falls to the ground as a liquid, flows a short distance, and solidifies. This produces a renewable deposit of mineral sulfur that local people mine and carry to a local sugar refinery that buys it.
Miners walk up the flank of the mountain and then descend dangerous rocky paths down the steep walls of the caldera. Then, using steel bars, they break sulfur from an outcrop, load their baskets, and make the return trip to the refinery. Miners make one or two trips per day carrying up to 200 pounds of sulfur. The refinery pays them based upon the weight of sulfur that they deliver. The rate of pay amounts to a few dollars per trip. Ambitious and physically fit miners can make two trips per day.
Miners have carried hundreds of sections of pipe up the mountain. These have been used to capture the gases produced by numerous vents and route them to a single area where their sulfur spills onto a level work area. This makes collection more efficient and safer for the miners.
Sulfur mining at Kawah Ijen has its hazards. The steep paths are dangerous, the sulfur gases are poisonous — young boys look like old men, and the average life expectancy of about 47. Occasional gas releases or phreatic eruptions have killed many miners.
Should the aluminum sheet be dipped into the lake for 20 minute, it’ll immediately blister and soon turn as thin as a piece of cloth.
Some miners don’t fully realize that the air they breathe is dangerous to their health. Many use old respirators, unable to afford replacement filters. As a protection from poisonous fumes miner use wet strips of cotton. They bite into a wet fabric with their teeth and breathe through it. Almost everyone smokes. Miners say that smoking helps them to obliterate a smell of sulfur that is nearly impossible to bear.
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