Travellers flock to the Indonesian island of Java to see the magnificent Kawah Ijen volcano – but what they don’t expect to find is the stunning turquoise-hued caldera lake at the volcano’s summit. To add to the drama, bright, citrine-coloured stones and billows of white gasses surround the 1km-wide aquamarine lake in a spectacular show.

One element is responsible for the entire, striking scene: sulphur. The magma chamber below the volcano pours sulphuric gases into the lake. Combined with a high concentration of dissolved metals, the gases turn the water a brilliant shade of blue. They also render the Ijen crater-lake the world’s largest highly acidic lake with a pH of 0.5.

That same chamber blasts a continuous stream of sulphuric gas from lakeside fumaroles that swirl around the lake. When the gas condenses and falls to the ground, it dyes the lake’s surrounding stones a shocking shade of electric…