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The Sunda Strait separates the islands of Sumatra and Java with the volcanic island Krakatau lying between them. Krakatau erupted violently in 1883, destroying two-thirds of the island and leaving a large caldera under the sea. This cataclysmic explosion was heard as far away as the island of Rodrigues near Mauritius (approx. 3000 miles or 4800 km away).[1] A new parasitic cone, called Anak Krakatau (or the child of Krakatau), rose from the sea at the center of the caldera in 1930.[12] The other Krakatau islets from the 1883 eruptions are known as Sertung, Panjang and Rakata.

Java is a relatively small island compared to Sumatra, but it has a higher concentration of active volcanoes. There are 45 active volcanoes on the island excluding 20 small craters and cones in the Dieng volcanic complex and the young cones in the Tengger caldera complex. Some volcanoes are grouped together in the list below because of their close location. Mount Merapi, Semeru and Kelut are the most active volcanoes in Java. Mount Semeru has been continuously erupting since 1967.[13] Mount Merapi has been named as one of the Decade Volcanoes since 1995.[14] Ijen has a unique colorful caldera lake which is an extremely acidic natural reservoir (pH<0.3).[15] There are sulfur mining activities at Ijen, where miners collect highly concentrated sulfur rocks by hand.