In the hours before dawn, Kawah Ijen burns with electric-blue flames. Sulphur gas seeps out of cracks in the rock and catches fire as it meets the air. The flames glow only in the dark and vanish the moment the sun rises. The volcano stands at the far eastern tip of Java, and its crater holds the largest highly acidic lake on Earth: a milky-turquoise pool roughly a kilometre long and 200 metres deep, more corrosive than car-battery acid at a pH near zero. The colour comes from the water itself, which is loaded with sulphuric and hydrochloric acid and dissolved metals. The escaping gas reaches up to 600 degrees and the flames can stand five metres high. Only a handful of volcanoes anywhere do this. The summit stands at 2,769 metres on the border of Banyuwangi and Bondowoso, and its basin became a UNESCO Global Geopark in 2023.
Ijen is also a working sulphur mine, and the job is among the hardest done on any volcano. Since the 1960s, miners have chipped solidified sulphur from the crater floor by hand, then shouldered wicker baskets of 75 to 90 kilograms up the steep path to the rim, paid by the kilo and breathing acrid fumes with only a rag across the face. Most travellers climb the other way — setting out from the Paltuding trailhead around two in the morning, first to the flames at the crater floor, then back to the rim to watch sunrise turn the lake to gold-green glass. The gas is dangerous, so a mask is now required, and the mountain closes for conservation on the first Friday of each month. The flames disappear as soon as the sky begins to lighten, so the whole display lasts only the last few hours of the night.