Kawah Ijen is a volcano in East Java where the sulphur gas leaking out of the crater floor catches fire in the air and burns electric blue. It is not lava, and it does not stop at sunrise; the flames are simply too faint to show against daylight. People walk up in the middle of the night to see anything at all. You come down into the crater over loose rock in the dark, and the first sign is not light but noise. The vents roar under pressure, loud enough that you raise your voice, sulphur crusts everything yellow, and the heat arrives at your face in waves. Some of the burning gas condenses back into molten sulphur and runs downhill still blue. The plume swings with the wind, and when the guides move you move with them.
The other people down there are at work. About two hundred miners chip the cooled sulphur into bright yellow slabs and stack them in twin wicker baskets slung from a shoulder pole. A full pair weighs seventy-five to ninety kilograms, and they carry it up the same path you came down and on to a weighing station below. They are paid by the kilo, and the sulphur ends up bleaching sugar and tipping matches a long way from here. Climbing back out before first light gets you to the rim for the other half of it. A kilometre-wide pool of turquoise acid sits cupped in bare rock with steam drifting off it, close to pH zero and the largest of its kind on Earth. As the sun clears the ridge the surface turns to gold-green glass and deepens by the minute. That water does not stay in the crater either. It drains out as the Banyu Pahit, the bitter water, a river still sour many kilometres downstream, where farmers irrigate their fields with water sharp enough to sting.