8.0583°S, 114.2425°E.
Active stratovolcano in East Java, Indonesia, containing the largest acidic crater lake on Earth. The lake is 700 metres in diameter, 200 metres deep, with a pH of 0.13–0.5 — comparable to battery acid — and contains an estimated 36 million tonnes of dissolved sulfur and metal salts. Sulfur condensation pipes installed by Dutch colonial mining in 1968 produce ceramic-pure native sulfur that local miners hand-carry up the crater wall in baskets weighing 70–90 kg, twice daily, for less than $20 per haul. The combustion of the sulfur-rich gases at the vent produces electric-blue flames visible only at night, reaching 5 metres tall as the gas burns at 600 °C. The phenomenon was first photographed by Olivier Grunewald in 2008.
Men carry battery-acid sulfur out of an active crater on their backs by the basket — and at night the gas they pass through burns blue.