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 Kelud are the most active volcanoes in Java. Mount Semeru has been continuously erupting since 1967. Mount Merapi has been named as one of the Decade Volcanoes since 1995. Ijen has a unique colourful caldera lake which is an extremely acidic natural reservoir (pH<0.3). There are sulfur mining activities at Ijen, where miners collect highly concentrated sulfur rocks by hand.