Ijen

Local workers hike up the side of the mountain and down into the crater at the top to harvest its sulfur—a byproduct of the gas that escapes from the volcano’s vents and collects near the shores of an acidic lake at the crater’s center. The chemical is used in industry worldwide, from making matchsticks to vulcanizing rubber, but Ijen’s sulfur goes mostly to local factories, which use it to bleach sugar.

2015 February 25, Kevin McElvaney, “Inside Indonesia's Ijen Volcano”, in The Atlantic