There are plenty of natural treasures hidden away in the most unsuspecting places on Earth. One of them is an Indonesian sulfur mine, Kawah Ijen, that produces stunning, spectral blue lava. The images of this mine are so breathtaking, I could just stare at them for hours.

Kawah Ijen is a part of the Ijen volcano complex – a group of stratovolcanoes in East Java, Indonesia – with an active crater that’s 200 meters deep. The complex is also home to the world’s largest turquoise-colored acidic lake, full of sulfuric acid. The lake is a sulfur mining site; miners carry sulfur-laden baskets by hand from the crater floor.

The miners work at night to double their meagre income, but they don’t have to worry about the dark. Kawah Ijen, an ordinary rocky crater by day, transforms into a stunning display of electric blue light at night.

Photo: Olivier Grunewald

As the sun sets, an eerie incandescence rises from the depths of the crater. The high-temperature liquid sulfur that flows from the edge of the lake flares up in blue flames that reach up to 5 meters in height. Although the conditions at the lake aren’t hot enough for the sulfur to self-combust, it just turns molten when miners drop their torches.

Photo: Thorsten Boeckel

The miners at Kawah Ijen work in pretty bad conditions – with little or no safety equipment. They monitor the molten sulfur as it flows out of pipes (from inside the volcano). Then they gather the coagulated sulfur, load it up and transport it to the side of the crater. The crystals are sold at about 680 rupiahs per kilogram (that’s about 5 US cents). The miners haul up about 80 to 100 kilograms per load, two loads every 24 hours. The sulfur mined at Kawah Ijen is among the purest in Indonesia, and used in the food and chemical industry.

Photo: Martin Rietze

I think the crater is amazing, but its beauty is a little bit marred by the treacherous working conditions of the miners. Many of them are grossly underpaid. They end up losing their health, and sometimes even their lives in the process of getting out the sulfur from the suffocating mines.

Photo: Olivier Grunewald

If you’re interested in visiting Kawah Ijen, you must know that the mine isn’t such an easy place to be in. One tourist wrote on her blog: “From an active vent in the volcano, gases billow out. And when the wind blows, these dense fumes head straight to your lungs, leaving you coughing and anxious to find cover.”

Photographer Olivier Grunewald tried to capture these otherworldly blue flames in 2008, and lost two lenses and a camera in his quest for the perfect photos. He wore gas masks for the shoot and had to discard his clothing afterward. If you still want to go, make sure you don’t step into the lake – it’s pure acid.

Sources: Yahoo News, A Cruising Couple