The geopolitical foundation of modern Indonesia is the seas and ports that integrate the country's islands that stretch from the Indian Ocean to the Pacific. Of Indonesia's more than 17,000 islands, about 6,000 are populated by more than 300 ethnolinguistic cultures. The Dutch first consolidated large parts of modern Indonesia by the 19th century, and maintaining political unity of its culturally diverse islands poses the nation's primary geographic challenge. Indonesia's main island, Java, is home to about 60 percent of the country's population, with its largest ethnic group being the Javanese. Java is also Indonesia's agricultural heartland, producing coffee, sugarcane, rubber and rice. A spine of east-west volcanic mountains runs along the southern reaches of the island. These mountains orient Java's population to the north, toward the Java Sea. The Java Sea is the core of Indonesia, with the Karimata Strait, the Makassar Strait and the Flores Sea linking the main island of Java to its outer islands, stretching from Sumatra to Sulawesi to western New Guinea. These islands have been critical sources of oil, natural gas, copper and tin. Indonesia's disconnected landmasses and vast maritime stretches present difficulties and limitations to centralized control, as seen with the independence of East Timor in the 1990s and ongoing separatist concerns in Aceh and Irian Jaya. Holding this island nation together has required a strong central authority to balance regional aspirations and national interests — although it has tried to balance tight control with empowerment of regions in recent decades. Ultimately, Indonesia is burdened by the conflicting requirements of managing the populations of each island while expending resources to control its maritime core.