Indonesia is famous for hosting some of the world's most powerful volcanoes. Krakatoa, located on an island in the Sunda Strait between Java and Sumatra in Indonesia, is well known because of its 1883 eruption, which generated the loudest sound historically reported: it was distinctly heard even in the Australian city of Perth (approx. 1930 miles or 3100km), or the island of Rodrigues near Mauritius (approx. 3000 miles or 4800 km). 36,417 were killed, mostly by the tsunamis which followed the explosion.
The 1883 eruption of Krakatoa was equivalent to an explosion of 200 megatons of TNT, about 13000 times the yield of the Little Boy bomb which devastated Hiroshima, Japan. Sea waves caused by the eruption were recorded as far as the English Channel.
Still, the most violent volcano on Earth is considered Tambora (2,865m or 9,550ft tall) from the Sumbawa Island (Indonesia). On 10th of April 1815, the mountain blew out ejecting its peak into the atmosphere at a height of 1,250m (4,160ft)! The cataclysm was marked with 7 on the VEI scale and at the moment killed 12,000 people. Subsequently, 80,000 people died of hunger.
About 150-180 cubic kilometers of debris were projected into the stratosphere, dimming the sky and depriving the Northern Hemisphere of summer.
But perhaps the most amazing Indonesian volcano is Kawah Ijen (2,600m or 8,660ft tall), the "Green Crater" from western 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.