Over the past month, the web has come alive with French photographer Olivier Grunewald's spectacular photos of Indonesia's Kawah Ijen volcano. Snapped during shooting of a new documentary he's releasing with the president of Geneva's Society for Volcanology, Régis Etienne, the photos—taken without the aid of any filter or digital enhancement—showcase the volcano's amazing electric blue glow.

Little of the web coverage, though, has enlightened readers on the scientific principles at work. "This blue glow, unusual for a volcano, isn't the lava itself, as unfortunately can be read on many websites," Grunewald says. "It is due to the combustion of sulfuric gases in contact with air at temperatures above 360°C."

In other words, the lava—molten rock that emerges from the Earth at ultra-high temperatures—isn't colored significantly differently than the lava at other volcanoes, which all differ slightly based on their mineral composition but appear a bright red or orange color in their molten state. But at Kawah Ijen, extremely high quantities of sulfuric gases emerge at high pressures and temperatures (sometimes in excess of 600°C) along with the lava.

Exposed to the oxygen present in air and sparked by lava, the sulfur burns readily, and its flames are bright blue. There's so much sulfur, Grunewald says, that at times it flows down the rock face as it burns, making it seem as though blue lava is spilling down the mountainside. But because only the flames are blue, rather than the lava itself, the effect is only visible at night—during daytime, the volcano looks like roughly any other.

"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."

Grunewald first heard about the phenomenon from Etienne, who visited the volcano in 2008 with an Indonesian guide. After being shown Etienne's photo featuring a child miner's silhouette surrounded by the blue glow, he was struck by the idea of photographing the mountain's sulfur miners working at night.

These miners extract sulfuric rock—formed after the blue flames have gone out and the sulfur gas has cooled and combined with the lava to form solidified rock—for use in the food and chemical industries. "To double their meager income, the hardiest of these men work nights, by the electric blue light of the sulfuric acid exhaled by the volcano," Grunewald says. Some of the workers are children, seeking to support their families by any means possible.

They carry rock-filled baskets by hand down the mountain, selling it for about 680 Indonesian rupiahs per kilogram, the equivalent of about six cents. In a country where the median daily income is about $13, many work overnight to supplement their income. Grunewald estimates that these nighttime miners can mine and carry between 80 to 100 kilos over the course of twelve hours of work—about $5 to $6.

Grunewald and Etienne produced the documentary partly to bring attention to these harsh working conditions. Most of the miners do not have gas masks (which the photographers wore throughout shooting and distributed to miners afterward), and suffer from health problems due to prolonged exposure to sulfur dioxide and other toxic gases.

Shooting these striking photos—some taken just a few feet away from the flames—was far more physically demanding than most of Grunewald's previous projects of landscapes and wildlife. "The main problem was the acidic gases that whirled constantly in the crater," he says. "The night seriously increased the difficulty as well, because it became almost impossible to see when dense gases arrived—at times, we were stuck in gas plumes for over an hour without being able to see our hands."

Just 30 nights in the crater, distributed over six trips, were enough to show Grunewald how destructive the environment of these mines can be. "During my first trip, I lost a camera and two lenses that had been corroded by acid," he says. "After we got back home, it took up to three weeks for our skin to lose the smell of sulfur."

His photos make the blue flames appear dramatically beautiful, even surreal. But for the miners that spend months or years at the volcano, the sulfur dioxide is quite real, and the health effects of chronic exposure—throat and lung irritation, difficulty breathing and a propensity for lung disease—can be devastating.