Java is a island in Indonesia, world’s 13th largest island. Java is almost entirely of volcanic origin; it contains no fewer than thirty-eight mountains forming an east-west spine which have at one time or another been active volcanoes. The highest volcano in Java is Mount Semeru (3,676 m). The most active volcano in Java and also in Indonesia is Mount Merapi (2,968 m). The island’s longest river is the 600 km long Bengawan Solo River.