West of Gunung Merapi is the Ijen volcano, which has a one-kilometre-wide (0.62 mi) turquoise-coloured acidic crater lake.

The lake is the site of a labour-intensive sulfur mining operation, in which sulfur-laden baskets are carried by hand from the crater floor.

[3] Workers earn around US$13 per day and, once out of the crater, still need to carry their loads of sulfur chunks about three kilometers to the nearby Paltuding Valley to get paid.

The active crater at Kawah Ijen has a diameter of 722 metres (2,369 ft) and a surface area of 0.41 square kilometres (0.16 sq mi).

Escaping volcanic gases are channeled through a network of ceramic pipes, resulting in condensation of molten sulfur.

[citation needed] The sulfur, which is deep red when molten, pours slowly from the ends of these pipes and pools on the ground, turning bright yellow as it cools.

Miners carry loads ranging from 75 to 90 kilograms (165 to 198 lb) up 300 metres (980 ft) to the crater rim, with a gradient of 45 to 60 degrees, and then 3 kilometres (1.9 miles) down the mountain for weighing.

[14] Ijen and its sulfur mining was featured in the 1991 IMAX film Ring of Fire, and as a topic on the 5th episode of the BBC television documentary Human Planet.