What’s spookier than the ‘largest acid cauldron on Earth?’ In East Java, Indonesia, lies the Kawah Ijen Crater Lake – the world’s largest acidic lake, a chilling spectacle perfect for Halloween.

The images here have been captured by the Copernicus Sentinel-2 mission which provides optical images of Earth’s surface. In the true-colour image in the left, 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 image on the right, on the other hand, takes on a horror-style palette for Halloween. It has been processed including a specific band from Sentinel-2 that is often used for coastal water and ocean colour observation. By using this band combination, the Kawah Ijen Crater Lake shows a higher reflectance compared to their surroundings, with the lake’s waters appearing electric blue.

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 images 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.