When photographer Reuben Wu heard about Indonesia’s Kawah Ijen volcano, curiosity quickly got the best of him. Located along the rocky coast of East Java, the steaming caldera hosts a viridian lake filled with hydrochloric acid, and is lined with streams of electric-blue fire. In fact, Ijen is the largest blue-flame generator on Earth.

It isn’t lava that gives the caldera its blue hue – a passerby would actually have a difficult time picking out Ijen lava from any other – but rather pockets of pressurized sulfuric gas, which spew out of cracks in the volcanic crust at an impressive 1,112°F (600°C). When the hot gas comes into contact with our oxygen-rich atmosphere, it ignites, sending flames as tall as 16 feet (5 meters) into the night sky. As the gas cools, some of it condenses into liquid sulfur, which continues to burn as it oozes its way down the cliff face.

It takes nearly three hours of steep climbing just to reach the base of the lake, but Wu was determined to make the trek. “It was my love for volcanoes which brought me to East Java,” he says. “I think this childhood obsession was rivaled only by space travel, and dinosaurs. I was specifically interested in visiting Kawah Ijen to see these molten sulfur streams at night. I had discovered the phenomenon a number of years back on a vulcanology website and have wanted to go since then.”

You may have seen our recent post about the science of Game of Thrones-style wild fire. Coincidentally, what’s happening here is very similar: the otherwise-yellow sulfur burns blue because of chemistry. Orbiting the atoms of every element are negatively charged electrons. When an element is ignited, the energy from the fire is absorbed, exciting those electrons. This takes the atoms, in this case of sulfur, out of what’s called “ground state,” the state at which they’re most relaxed and stable. Like everything in nature, atoms don’t like to be unstable, so to right themselves, they release all of that stored energy in the form of particles of light, called photons.

The amount of energy in a photon determines its color: red is the lowest, increasing through the rainbow of orange, yellow, green, blue, indigo, and finally violet. Ijen burns a bright blue because sulfuric fires put out an immense amount of energy.

“The gas was hot, yellow and poisonous,” recalls Wu. “Most of the time inside the crater I was engulfed in a plume of it, which was disorienting, and made it difficult to breathe and see. Operating a camera became an effort, but I enjoy going to strange and unique places, where normal rules of terrain do not apply. The volcanic landscape changes so quickly. Mountains can appear overnight, and lakes can disappear. It really feels like you can see these forces in action here.”

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IMAGES: Reproduced with permission from Reuben Wu