Kawah Ijen is a volcano in East Java, Indonesia, holding the largest acidic crater lake on Earth, a kilometre-wide pool of turquoise water close to pH zero, and before dawn the sulphur gas escaping beside it burns with an electric-blue flame. The flames are the reason people climb it in the dark. You come down into the crater over loose rock, and the first sign is not light but noise: the vents roar under pressure, loud enough that you raise your voice, and then the blue appears, flickering along the rocks where sulphur gas meets the air and ignites at over 300 degrees. It is not blue lava; it is burning gas, and it can only be seen at night, which is why the walk starts around two in the morning. Gas masks are handed out at the rim, and the smoke shifts with the wind.
The other people in the crater are working. Around 200 miners hack cooled sulphur from the vents by hand, bright yellow slabs that they stack in twin wicker baskets slung from a shoulder pole, seventy to ninety kilos at a time, and carry up the crater wall and three kilometres down the mountain to a weighing station; the same path you climb for the view is their commute, and it is some of the hardest paid work on any volcano. Climbing back out of the crater before first light, you reach the rim as the whole lake opens below you for the first time, a turquoise disc cupped in bare grey rock with steam drifting off it, and beyond the crater the cones of the Ijen plateau standing in cloud. The lake drains into the Banyu Pahit, the bitter water, a river that stays acidic far downstream, where farmers still irrigate crops with water sharp enough to sting. Ijen gives you a blue flame, an acid lake and a sulphur porter in one night, and the porter is the one you remember.