The island of Java is Indonesia's heartland, the nation's cultural capital, its political and economical centre, although it covers only 7 percent of the total land mass. Java is an island of contrasts: landscape of small villages and modern cities, western architecture and traditional Indonesian culture, nightly activities divided between delightful wayang kulit puppet performances and all-night dancing at discos in Jakarta, courtly dance and gamelan is performed with a subtle fervour while others pour their energy into working the rice fields. This is the most fertile, the most productive, and probably the most densely populated island in the world. With over 115 million people living in the area of 132,187 km2, the average population density is 850 persons per square kilometre. And although there are cities with over million inhabitants, the island is still predominately rural. Volcanoes are the very core of Java. They have shaped the landscape and provided the source for Java's rich soils. The volcanoes form an uneven stripe running the whole length of the island - one of the most active segments in the Pacific "Ring of Fire". The volcanoes are more strongly crammed in the west, where they make a complex of highland plateaus and valleys. To the east, they are more broadly spaced, creating a series of steadily sloping valleys that are perfectly suitable to rice field terracing. Java and Bali together have 37 volcanoes officially listed, of which 23 have been active since 1600. The highest peak is Mount Semeru at 3,676 metres. The most famed volcano is Krakatau in the Sunda Straits, who disastrously erupted in 1883.