The Ijen volcano is 2250 meters high and has a crater lake with a very high sulfur content, which makes the water turn turquoise. With a diameter of 1 kilometer and a depth of 200 meters, it is the largest acidic crater lake in the world.
At night, a blue fire can sometimes be seen in the crater of the volcano. It is the largest blue fire area on Earth. The flames can reach a height of five meters and reach a temperature of 600 degrees Celsius. The blue fire arises because sulfuric acid escaping from the soil ignites after it comes into contact with the oxygen in the air.
The government requires tourists to drive a jeep to the beginning of the park. It is a 2-hour walk from the park entrance to the crater lake. For tourists it is mandatory to do this under the supervision of a guide. The climb to the top is quite tough. Due to the height it can get cold (be sure to bring a jacket). You can rent a gas mask for the sulfur fumes.
Sulfur has been extracted at the edge of the lake since 1968. The solidified sulfur is chopped up and carried down by about 300 carriers in baskets. The carriers carry between 75kg and 90kg first up from the crater and then 3km down. They make the trip twice a day.
On the westside of the crater lake a dam was built around 1920 to prevent the toxic water from the lake from flowing over the lower farmland. This dam itself is difficult to reach.
Since National Geographic, the BBC and Al Jazeera broadcast documentaries about the extraordinary blue fire, tourism to the volcano has increased. It can be busy in the high season, especially on weekends.