Nature photographer Olivier Grunewald and cameraman Régis Etienne first traveled to East Java’s 8,660 foot tall (2,600 meter) Ijen volcano in 2008, returning several times to capture Le Mystère des Flammes Bleues, the blue molten sulfur fires found in the crater. Turn on the english captions for the video above. From National Geographic:

“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.”

River of sulphur near the acid lake of the Kawah Ijen volcano in Indonesia

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“The vision of these flames at night is strange and extraordinary,” Grunewald says. “After several nights in the crater, we felt really living on another planet.”

Related photography by Grunewald at Boston.com’s The Big Picture, and by Reuben Wu at Colossal. Plus, another Kawah Ijen fact: The volcano’s turquoise-colored crater lake is the largest acid lake on Earth.

Watch this next: Aluminum foil boat floating on a sulfur hexafluoride sea.

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