Acid lake of Kawah Ijen, Indonesia

In East Java, Indonesia, lies the Kawah Ijen Crater Lake – the world’s largest acidic lake. In this image captured by the Copernicus Sentinel-2 mission we can see Kawah Ijen with its striking turquoise blue waters.

Although seemingly inviting, the lake is filled with a high concentration of sulphuric and hydrochloric acids, as well as dissolved minerals. Though this deems the lake unswimmable, it makes it incredibly easy to spot from space. The water in the Kawah Ijen Crater Lake has pH values as low as 0.5, similar to the strength of car battery acid. 

The lake's unnaturally high acidity is not its sole frightening characteristic. It also emits hot, flammable sulphurous gases that ignite as they enter Earth’s oxygen-rich atmosphere. These then burn with an eerie, blue flame, creating an enchanting nighttime spectacle.

Also seen in the image is the Raung, or Gunung Raung Volcano, one of the most active volcanoes on the island of Java. With an impressive height of 3332 m, it looms in the immediate southwest vicinity.