Kawah Ijen

The Ijen volcano complex sits on the eastern tip of Java, flanked by coffee and rubber plantations on its lower slopes and bare volcanic rock near the summit. At an elevation of 2,386 metres, the crater holds a lake of extraordinary turquoise — roughly one kilometre across, 200 metres deep, its colour the result of dissolved volcanic gases and minerals creating sulfuric acid concentrations so extreme the water can dissolve iron and cloth. It is the largest acidic crater lake in the world.

By day, Kawah Ijen is a spectacle of chemistry: the lake steams continuously, fumaroles emit columns of sulfur dioxide, and the deposits at the water's edge are vivid yellow where elemental sulfur has crystallised from the gas. Miners — some who have worked this crater for decades — descend the inner wall, fill their baskets with chunks of bright yellow sulfur, and carry loads of 80 to 90 kilograms back up the steep path. It is some of the hardest manual labour still practiced anywhere on Earth.

After dark, something rarer happens. When sulfuric gases escaping from the volcanic vents ignite on contact with the atmosphere, they burn. But they burn blue — an intense, electric blue that has nothing in common with the orange-yellow of ordinary fire. The flames reach up to five metres, run in rivers down the crater's inner walls, pool in the rock depressions. The miners work inside this light, their gas masks and headlamps moving through a landscape that looks nothing like Earth.

The blue fire is not unique to Ijen — sulfuric gases ignite blue wherever they combust — but Ijen is the only place where the phenomenon is visible at a scale and accessibility that allows it to be witnessed without extraordinary difficulty. The fire burns continuously. On moonless nights, the crater of Kawah Ijen is blue.