Sitting in the heart of the Ijen crater is a sky-blue lake approximately one kilometer across. It is the biggest crater lake in Java. The colors of the lake contrast with the sterile moonscape of the rest of the crater, and at first glimpse you might think this a beautiful scene. It is 200 meters deep and contains about 36 million cubic meters of steaming acid water, shrouded in a smelling, swirling sulfur cloud.
During the early morning hours you can see the so-called "blue fire," which is pretty spectacular.
Around the edges of the lake is a series of vents where the volcanic gases escape. Local miners have placed ceramic pipes over these vents and as the gases travel down the pipes, they condense and turn into liquid sulfur. This quickly cools and solidifies. This bright yellow residue, used in cosmetics and medicines, is harvested by miners using only the most basic of tools.
The view of sulfur miners who climb and descend the crater is amazing. A man puts up to 90 kg of yellowish stone into his basket before he descends the mountain slope to sell his load — and this usually at least twice a day.