The lake of ijen volcano complex

The Ijen volcano complex at the eastern end of Java consists of a group  of small stratovolcanoes constructed within the large 20-km-wide Ijen  (Kendeng) caldera.  The north caldera wall forms a prominent arcuate  ridge, but elsewhere the caldera rim is buried by post-caldera  volcanoes, including Gunung Merapi stratovolcano, which forms the 2799 m  high point of the Ijen complex.

At the time of publication, it represented the best available science.  The turquoise lake in the crater of the Kawah Ijen volcano looks serene  and inviting. It also happens to be the world's largest acidic lake. The water in the crater lake has a pH less than 0.3 on a scale of 0 to 14 (7 is neutral).

In their search for “devil’s gold,” as they call it, about 300 miners  make a daily climb two miles up the mountain, then head downward more  than 900 yards into the volcano, where the sulfur crystals form. Most  work without any protection in darkness and stifling heat—all the while  breathing sulfurous gas that burns their lungs and makes tears stream  from their eyes.

To retrieve the sulfur, they attack it with a  metal pole to break it into slabs.