Kawah Ijen and the blue fire

Blue flames five metres high, and men carrying 90 kilos out of the fumes

Kawah Ijen lies inside the great Ijen caldera, some 20 kilometres wide, on the border of Banyuwangi and Bondowoso regencies at the eastern end of Java, and at night its crater burns blue. Sulfurous gas escapes from cracks at up to 600°C and burns, throwing up electric-blue flames as tall as five metres; some of the gas condenses and keeps burning as it runs down the rock like liquid. Local people call it api biru, blue fire. Ijen has the largest area of blue flame in the world, and Ethiopia's Dallol is the only other place it can be seen.

The crater holds a turquoise lake nearly a kilometre across and 200 metres deep, the largest highly acidic crater lake in the world. A scientific expedition in 2001 measured its pH below 0.3, and in July 2008 the explorer George Kourounis paddled a rubber boat onto it and found 0.13 at the centre, thanks to a high concentration of sulfuric acid. Its water feeds the Banyupahit River, carrying acid and metals downstream.

The flames have made Ijen a destination. Since National Geographic wrote about them, tourists have been coming in growing numbers for midnight hikes: about two hours up to the crater rim, then 45 minutes down to the shore to watch the fire before dawn. As one local warned the Jakarta Post, the path is steep in the dark, and the sulfurous fumes hurt the eyes and make breathing hard.

The other spectacle is human. At a vent on the lake's edge, volcanic gas is channelled through ceramic pipes until molten sulfur, deep red, drips from the ends and hardens bright yellow. Some 200 miners break it into large pieces and carry it out in baskets of 75 to 90 kilograms, up a couple of hundred metres of steep crater wall and then about three kilometres down the mountain to the weighing station, often twice a day, for pay that worked out around US$13 a day in 2010. Many work with little protection from the fumes and complain of breathing problems. Together they take out about 14 tonnes a day, perhaps a fifth of what the vent deposits. The sulfur ends up bleaching sugar, making matches and fertiliser and vulcanising rubber.

The miners' work has been filmed for IMAX, the BBC's Human Planet and the documentaries War Photographer and Workingman's Death. Ijen became part of a UNESCO Global Geopark in 2023.