The volcano contains the world’s largest acid lake

Kawah Ijen is a giant active volcano on the island of Java with a crater containing the world’s largest acid lake. The water has a pH lower than 0.3 in many parts of the lake due to hydrothermal fluids rising from within the Earth’s crust, filling the water with minerals as well as sulfuric acid and hydrochloric acid.

A pH of 0.3 is equivalent to battery acid, the solution that generates and stores electrical energy in the car. For comparison, stomach acid has a pH of 1.5 – 2 and the pH of lemon juice is 2 – 3. But despite conditions that can instantly dissolve human skin, Kawah Ijen’s acid lake is still home to small bacterial populations, according to a 2006 study.

The green-blue color of Kawah Ijen crater lake is so bright that it can be seen from space. Minerals and acids in the crater lake make the water turquoise. They originate from a hot magma chamber beneath the volcano, which last erupted in 1999. The volcano is 2,769 m high and the crater is 700 x 800 m wide.

Regular rains replenish Kawah Ijen’s acidic lake with new water, but the new water is immediately erosive due to the continuous release of gas at the bottom of the lake. When the crater is full, water flows into a stream on the western side of the volcano into the Banyupahit river basin.

If the acidic water of the crater lake wasn’t strange enough, Kawah Ijen also emits flammable sulfur gas when exposed to oxygen in the air. When these gases ignite above the volcano, they create a blue flame that is almost invisible during the day, but very eye-catching at night.

Normally, the sulfur in the gas condenses after ignition, forming a liquid that flows a short distance along the volcano before solidifying into a yellow deposit. Local people mine this ore, breaking up sulfur blocks to sell to the area’s sugar refinery. There, they are used to bleach sugar. Mining comes with great risks, including toxic sulfur fumes, frequent gas explosions and treacherous volcanic passages.