For several years Paris-based photographer Olivier Grunewald has been documenting the Kawah Ijen volcano in Indonesia, where dazzling, electric-blue fire can often be seen streaming down the mountain at night.

“This blue glow—unusual for a volcano—isn’t, of course, lava, as unfortunately can be read on many websites,” Grunewald told National Geographic in an email about Kawah Ijen, a volcano on the island of Java.

The glow is actually the light from the combustion of sulfuric gases, Grunewald explained.

Those gases emerge from cracks in the volcano at high pressure and temperature—up to 1,112°F (600°C). When they come in contact with the air, they ignite, sending flames up to 16 feet (5 meters) high.

Some of the gases condense into liquid sulfur, “which continues to burn as it flows down the slopes,” said Grunewald, “giving the feeling of lava flowing.”

Cynthia Werner, a research geologist with the U.S. Geological Survey (USGS) at the Alaska Volcano Observatory, told National Geographic that Grunewald’s photos show an unusual phenomenon.

“I’ve never seen this much sulfur flowing at a volcano,” she said.

Werner noted that forest fires in Yellowstone National Park have caused similar “rivers,” as heat from the blazes melted the sulfur around hydrothermal vents.

“When you go to Yellowstone, you can see their traces as black lines,” she said.

According to Werner, it’s relatively common to find molten sulfur around volcanic fumaroles (hot vents). The mineral has a relatively low melting point of 239°F (115°C), and the temperature at the hot vents often exceeds that.

Blue volcanic fire was described in antiquity in Italy on the south slope of Mount Vesuvius and on the island of Vulcano, Grunewald said.

“Blue flames may also be observed at the base of the plume of erupting volcanoes, when ash explosions occur,” he added.

Grunewald did not use any filters to capture his images of the blue fire. The burning happens day and night, but it’s visible only in darkness.

Kawah Ijen volcano is the subject of a new documentary released earlier this month that was produced by Grunewald and Régis Etienne, the president of Geneva’s Society of Volcanology.

Kawah Ijen Crater Lake, at the top of the volcano, is the world’s largest such body of water filled with hydrochloric acid. In fact, it’s the acid that makes the water green.

Werner explained how the lake became so acidic: The volcano emitted hydrogen chloride gas, which reacted with the water and formed a highly condensed hydrochloric acid with a pH of almost 0.

The lake has a volume of 1.3 billion cubic feet (36 million cubic meters), or about 1/320 of the volume of Oregon’s Crater Lake.

As the burning gases cool, they deposit sulfur around the lake.

To speed up the formation of the mineral, a mining company installed ceramic pipes on an active vent near the edge of the lake, said John Pallister, a USGS geologist who has studied the volcano.

The pipes route the sulfur gases down the vent’s sloping mound. When the gases cool, they condense into liquid sulfur, which then flows or drips from the pipes and solidifies into hard sulfur mats.

After the solid sulfur cools, the miners break it up and haul it off the mountain on their backs.

“I have also seen the miners spraying water from a small pump onto the pipes to promote cooling and condensation,” said Pallister via email. “Sulfur stalactites sometimes form from the liquid sulfur dripping from the pipes. These are collected and sold to tourists.”

Pallister added, “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.”

Miners have been extracting sulfur here for more than 40 years. At times they work at night under the eerie blue light to escape the heat of the sun, and to earn extra income, Grunewald said.

The miners sell the sulfur for about 600 Indonesian rupiah per kilo (less than 25 U.S. cents per pound), said Grunewald. They can carry loads of 176 to 220 pounds (80 to 100 kilos) once a day—or twice if they work into the night.

When Grunewald photographs Kawah Ijen, he wears a gas mask as protection against toxic gases, including sulfur dioxide. “It is impossible to stay a long time close to a dense acid gas without a mask,” he said.

Pallister described the miners’ daily routine as “tough duty.” He has seen many of them using only wet cloths as gas masks.

Some of the miners do have gas masks that visitors have given them, said Grunewald, but they “have no money and no opportunity to change the filters.”

“I feel bad for these miners,” Werner said. When she and her colleagues work in Indonesia, “we usually bring gas masks and leave them there with the people we work with, because sometimes they don’t know that what they are breathing is harmful.”

Grunewald has also documented the blue glow on the Dallol volcano in the Danakil Depression, in the Afar region of Ethiopia near the borders of Eritrea and Djibouti.

The heat of magma sometimes ignites the sulfur dust in the soil, forming flames of electric blue.

“It is very rare to see that,” said Grunewald. “The powder of sulfur could burn for a few days.”

The depression is geologically active, with hydrothermal vents and sulfur springs, some of which are tourist attractions.

The Afar region is famous for having the world’s highest average temperature of 93°F (34°C), thanks in part to the volcanic activity

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