East Java molten sulfur
© Reuben Wu
Glowing blue rivers illuminate Java's Blue Fire Crater.

Visitors looking up at Kawah Ijen, a volcano on the Indonesian island of Java, are treated to a sight not often seen in the rest of the world: electric-blue rivers streaming out from underneath the volcano. Although it looks like bright blue lava, the streams are actually molten sulfur, which gives off a neon-like light at night.

"I've never seen this much sulfur flowing at a volcano," U.S. Geological Survey research geologist Cynthia Werner tells Brian Clark Howard for National Geographic.

Bright blue sulfuric flames aren't exactly rare around volcanoes. Sulfur has a relatively low melting point of 239 degrees Fahrenheit and small trickles and blue fires are often found near hot vents and during volcanic eruptions, Howard writes. However, Kawah Ijen is the largest "blue flame" area on earth, with jets of fiery sulfuric acid burning at over 1,000 degrees Fahrenheit and shooting 16 feet into the air, Christopher Jobson reports for Colossal.

East Java molten sulfur flow
© Reuben Wu
Kawah Ijen is a major source of income for many Indonesians who live at its base, but not because of the blue flames. The volcano's crater contains the world's largest body of water that is filled with hydrochloric acid, which in turn produces giant stalagmites of yellow sulfur that many locals depend on for their livelihood. While the gases spewing from the sulfur vents are highly toxic, many miners work with little to no protection, carrying hundreds of pounds of raw sulfur down the mountain for about $10 a day, Yenni Kwok writes for TIME LightBox.

"I have been told that the miners sometimes ignite the sulfur and/or sulfur gases to produce the blue flames that are so prominent in nighttime photographs," John Pallister, a USGS geologist, tells Howard.

The bright blue flames may be striking, their beauty comes at a real cost for those whose lives depend on Kawah Ijen's sulfur.

sulfur dust ignites in Ethiopia
© Olivier Grunewald
In Ethiopia's Danakil Depression, the sulfur dust in the soil of a hydrothermal vent ignites to form blue flames.