You've heard of "red hot" and "white hot" to describe searing temperatures. But what about "blue hot"?
That's the surreal hue of Indonesia's Kawah Ijen Volcano, which glows with an otherworldly "blue lava" at night. The mountain contains large amounts of pure sulfur, which emits an icy violet color as it burns, turning the rocky slopes into a hot (at least 239 degrees Fahrenheit), highly toxic environment.
Despite the dangers, photographer Olivier Grunewald captured the scene, along with a group of men who toil on the volcano at night, battling noxious gases to mine sulfur from the crater and carry it out by hand.
Miners carry between 176 and 220 pounds of sulfur chunks per trip and sell the pieces for around 2.5 cents per pound. Yahoo reports they average two loads every 24 hours, thereby doubling their salaries amid sulfurous flames that can reach 16 feet high.
The volcano is the subject of a new documentary -- produced by Grunewald and Régis Etienne, the president of Geneva's Society of Volcanology -- released earlier this month. (See the trailer, in French, below)
PHOTOS of Kawah Ijen's blue lava:
WATCH the trailer for Grunewald and Etienne's film, below: