A Lake of Battery Acid
The turquoise water of Kawah Ijen, located in East Java, is the largest highly acidic crater lake on Earth. This body of water has a volume of about 36 million cubic meters and fills a crater that is roughly 722 meters in diameter and 200 meters deep. The extreme acidity is due to a potent mixture of hydrochloric and sulfuric acids, with pH values measured as low as 0.5 at the edges and 0.13 in the middle, comparable to the strength of car battery acid. The water temperature hovers around 60 degrees Celsius (140 degrees Fahrenheit), heated by the magma chamber deep beneath the lake bed.
The lake's startling color is a result of its unique chemistry, specifically the high concentration of dissolved metals. Its hyperacidity originates from volcanic gases, like sulfur dioxide and hydrogen chloride, that emanate from fumaroles and dissolve into the water. This process creates a chemical environment so corrosive it can dissolve metal and cause severe skin irritation on contact. The acidic, metal-rich water flows out of the crater to form the Banyupahit River, which significantly impacts downstream ecosystems and is used by local farmers for irrigation, despite having a pH between 2.5 and 3.5. While the lake's water column itself is considered too extreme for most life, studies have found archaea and other extremophile microbes in the river system originating from it, showing life's ability to persist in harsh conditions.