The Philippines is an archipelago comprised of more than 7,000 islands lying about 500 miles southeast of mainland Asia. Its closest nation neighbors are Malaysia and Indonesia to its south. The island group consists of three geographical and cultural main island groups of Luzon, Visayas, and Mindanao. Of the 7,000 islands only 360 have areas of one square mile or bigger and only about 2,700 are named. Many are uninhabited and uncharted. Luzon, the largest island, has a land area of 40,420 square miles. The second largest island, Mindanao, has a land area of 36,537 square miles. The total land area of the Philippines is 115,830. The islands are primarily composed of coral and volcanic rock and feature mountainous country, interconnected volcanoes, lakes and rivers. Scientists say that the Philippine volcanoes are part of the Pacific "Ring of Fire" that includes the islands of Hawaii and the San Andreas Fault along the Pacific coasts of the American continent. Mount Pinatubo is known for its violent eruption in 1991. The most prominent mountain chain is the Cordillera Central running the length of Luzon Island which is composed of several large parallel mountain ranges. Run-offs from the constant forest rain running down the large mountain ranges feed several rivers and lakes that are important to the agricultural, fishery and commercial livelihood of Filipinos.