The Blue Fire Crater, as it is sometimes called, isn’t lava, but combusting sulfuric gas. The gas works its way through fissures to the surface, emerging at high pressure and extreme temperatures of up to 1,112 degrees Fahrenheit. Some of this gas ignites in flames up to 16 feet tall, and some becomes liquid sulfur that flows over the landscape.

Wu and his wife made the pilgrimage in July during a three-week vacation in Indonesia. The couple did some sightseeing, but the volcanoes were the star of the show. Wu took photos at Kawah Ijen and Mount Bromo a few hours away. “It was a holiday, but it was also quite a committed photo project for me at the same time,” he says.

Reaching the crater means a two-hour hike up a craggy trail, then a 45-minute descent to its bank. It’s a popular spot for tourists. The blue flames can only be seen at night, and many people visit in the wee hours and then watch the sunrise. But Wu was there at sunset, too, when there were fewer people around. “I prefer to see these places without crowds and the experience of being there at night alone is so much more intense than during the daytime,” Wu says.

Intense is an understatement. Wu’s photos perfectly capture the toxic beauty of an otherworldly landscape. Tiny yellow crystals of sulfur coat the rocks, contrasting with wisps of white smoke and steam drifting across a brilliant green lake.  The fumes can burn your lungs, so visitors wear masks. That can be disorienting, though, and Wu occasionally found himself lost in a cloud of sulfuric gas. “The plumes from the fumaroles drift around inside the crater, and it’s very difficult to navigate and orientate yourself, let alone focus on taking a photograph,” he says.

Aided by the light of a full moon, he made most of his photos with exposures as long as eight minutes using a tripod. He usually works in medium and large format photography, but for the sake of mobility in an active volcano shot in 35mm using only a couple of lenses. Given the difficulty of composing a shot and focusing the lens while wearing a mask, Wu “used The Force” and hoped for the best. “I remember being transfixed by this phenomenon,” he says, “the sight and sound of the sulfur overwhelmed everything else in my head for a while.”

It also overwhelmed his clothes. Wu says he’s washed them three times since his trip and they still carry the sulfuric reek of rotten eggs.

This article was originally published on Wired. Read the original article.