English: The turquoise lake in the crater of Indonesia's Kawah Ijen volcano may look serene and inviting, but it happens to be the world's largest acidic lake. The turquoise color comes from the range of materials dissolved in the water, including hydrochloric and sulfuric acids. The water in the lake has a pH less than 0.3 on a scale of 0 to 14 (7 is neutral). For comparison, lemon juice has a pH of 2; battery acid has a pH of 1.

The top image, first published by Earth Science Picture of Day, is a photograph taken by João Manuel Pires dos Santos on September 13, 2014. At the upper left, sulfuric gases are visible venting from fractures in the crater. The lower image was acquired by the Operational Land Imager (OLI) on Landsat 8 on August 22, 2013.

That acidity of the water affects the chemistry of nearby river ecosystems, including the river Banyupahit. Originating at the lake, the Banyupahit delivers acidic water to populated areas downstream. The river water local farmers use to irrigate crops has a pH between 2.5 and 3.5. The normal range for stream water, according to the U.S. Geological Survey, is between about 6 and 8.

Despite the presence of toxic gases around Kawah Ijen, workers mine sulfur by using a series of pipes installed under one of the volcano's active vents. Gas inside the pipes condenses into molten sulfur as it moves toward the surface, where it then cools and hardens.

See the photograph on Earth Science Picture of the Day at go.nasa.gov/1iaRRWc

Download a larger version of the satellite image at go.nasa.gov/1iaRUS0

See more photography by Pires dos Santos at

go.nasa.gov/1iaRUS4