In East Java, Indonesia, an explorer once paddled a rubber dinghy across a turquoise crater lake so corrosive its pH measured 0.13, stronger than battery acid. This is Kawah Ijen, home to the world's largest highly acidic crater lake and a volcanic complex that behaves like a different planet. Magma constantly degasses sulfur dioxide and hydrogen chloride, creating a continuously regenerating reservoir of sulfuric and hydrochloric acid that dissolves metals and gives the water its deceptively beautiful color.

At night the volcano emits towering electric blue flames as pressurized sulfuric gas auto-ignites at temperatures up to 600 degrees Celsius. Yet Ijen is also one of the most grueling sulfur mining operations on Earth. Local miners channel the gas through ceramic pipes to condense molten sulfur, then haul 75 to 90 kilogram loads up a st ...