Welcome to the largest, most highly acidic crater lake in the world
Image: Galyna Andrushko/Shutterstock

Inside a 20-kilometre-wide crater in East Java lies the Ijen volcano complex - a group of 'stratovolcanos' built up over hundreds of thousands of years from layers of hardened lava and other volcanic materials. Included in this group is the Kawah Ijen volcano, famous for the stunning electric blue glow that emanates from it thanks to a constant supply of sulphuric gas combusting as it makes contact with the air. This crater is also home to another natural beauty - a turquoise lake fringed with yellow that attracts locals to its banks for sulphur mining.

Mining sulphur here is a tough job. According to Indra Harsaputra at the Jakarta Post, workers earn around US$5.50 to $13 here per day, and that’s only after they carry their packs loaded with up to 90 kilograms of sulphur chunks for 3 kilometres to a nearby sugar refinery in Pultuding Valley. 

The miners extract the sulphur from an active vent at the edge of the lake. A series of ceramic pipes have been installed underneath the vent, and these channel volcanic gases up to the surface, which results in the condensation of molten sulphur. This deep red molten sulphur pours through the pipes into the vent on the surface, and turns bright yellow as it cools and sets. Around 200 miners work at the crater per day and extract about 20 percent of its daily sulphur supply.

While the clouds of toxic gases that surround the lake make it a pretty inhospital environment, the lake itself is incredibly beautiful. It’s 182 metres deep and contains 32 million cubic metres of turquoise green water. "The high acidity of the water gives it a unique, bitter flavour,” says Harsaputra at the Jakarta Post. It’s also not advisable to swim in it, for obvious reasons. But tourists have lately taken to boating down the 3-kilometre-long river that flows from the crater lake and into the Banyu Pahit River in East Java. "The river cruise is made more exciting by the presence of 15 waterfalls,” says Harsaputra. "Care should be taken to avoid falling into the river as the acidic water causes skin irritation.”

Unfortunately, the Banyu Pahit River - which ends up highly acidic and polluted with high amounts of metals such as iron and aluminium from the crater lake - is used to irrigate a large area of farmlands nearby. The river water has a pH level of between 2.5 and 3.5, according to a 2005 study led by Ansje J. Löhr, an expert in environmental chemistry at the VU University Amsterdam in the Netherlands. To put this in perspective, the NSW Environment Protection Authority in Australia recommends that the pH level of public drinking water be between 6.5 and 8.5.

Publishing in the journal Environmental Science and Pollution ResearchLöhr's team calls this ‘natural pollution’, because it hasn't been caused by human activity, and they warned that it continues to pose a serious threat to the environment, the local people’s health, and their agricultural production.