The most amazing Indonesian volcano is Kawah Ijen (2,600m or 8,660ft tall), the "Green Crater" from Eastern Java, which has a lake made of 36 million cubic meters representing a solution of sulfuric acid and hydrogen chloride, the most powerful existing acids.
On the edges of the lake, the fumaroles (volcanic gas eruptions) depose 4 tonnes of sulfur daily. Such acid lakes are also found on the volcanoes Kusatsu-Shirane (Japan) and Poas (Costa Rica), but the Indonesian lake is by far the largest acid lake on Earth, having a maximum depth of 212m (706ft). These lakes result from the mix of rainfall water with gases coming from the depths of the volcano.
The walls of the Kawah Idjen lake are light ocher, but the water has a turquoise color, with emerald reflexes. The temperature of the water is of 34o C, and sulfur bubbles float on the surface. The surroundings are covered by a sulfur powder. The smell is pungent and irritating, filled with sulfur dioxide. From place to place, sulfur pours at a temperature of 120o C, like bright red trails, which gradually solidify, turning lemon yellow.
Sulfur deposits on the edges of the lake
The lake contains 600,000 tonnes of hydrogen chloride, 550,000 tonnes of sulfuric acid, 200,000 tonnes of aluminium sulphate and 170,000 tonnes of iron sulphate.
People from the neighboring area extract sulfur from the crater manually - an extremely hard work. To increase efficiency, the workers build tunnels of stone and undulated plates to channel the sulfur-rich fumaroles. The sulfur then leaks, cools down and solidifies inside these improvised channels, which are subsequently broken using metal piles. The recovered stuff contains 99 % sulfur. The sulfur is made into pieces, loaded in baskets and transported on the men's back outside the crater.
In the irritating and corrosive atmosphere of the crater, people's only protection is a piece of fabric used for covering their mouths and noses. Each worker can transport 40 to 70kg (90 to 155 pounds) at once on the abrupt slopes of the volcano, using bamboo ladders where the slope is too steep.
Once on the top, the workers must descend to the weighing place. In one day, a man can carry up to 360 kg (750 pounds) of sulfur. The daily production of the exploitation is just of 4 tonnes, a derisory quantity, if we consider the fact that the crater harbors 30,000 tonnes of sulfur. The sulfur is transported to Banjuwangi, 37 km (23 mi) away and it will be used for vulcanizing rubber or refining sugar.