Kawah Ijen volcano in Indonesia, where dazzling, electric-blue fire can often be seen streaming down the mountain at night.

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

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

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Video credit: cameralucida

All photos credit: Photographer Olivier Grunewald