It now amounts to only a chain of limestone shoals remaining above sea level. Legends claim that it was passable on foot up to 1480 CE, until cyclones deepened the channel. Portions are still as shallow as , hindering navigation. The island consists mostly of flat to rolling coastal plains, with mountains rising only in the south-central part. The highest point is Pidurutalagala, reaching above sea level. Sri Lanka has 103 rivers. The longest of these is the Mahaweli River, extending . These waterways give rise to 51 natural waterfalls of or more. The highest is Bambarakanda Falls, with a height of . Sri Lanka's coastline is long. Sri Lanka claims an exclusive economic zone extending 200 nautical miles, which is approximately 6.7 times Sri Lanka's land area. The coastline and adjacent waters support highly productive marine ecosystems such as fringing coral reefs and shallow beds of coastal and estuarine seagrasses. Sri Lanka has 45 estuaries and 40 lagoons. Sri Lanka's mangrove ecosystem spans over 7,000 hectares and played a vital role in buffering the force of the waves in the 2004 Indian Ocean tsunami. The island is rich in minerals such as ilmenite, feldspar, graphite, silica, kaolin, mica and thorium. Existence of petroleum and gas in the Gulf of Mannar has also been confirmed, and the extraction of recoverable quantities is underway. Climate The climate is tropical and warm because of moderating effects of ocean winds. Mean temperatures range from in the central highlands, where frost may occur for several days in the winter, to a maximum of in low-altitude areas. Average yearly temperatures range from to nearly . Day and night temperatures may vary by to . The rainfall pattern is influenced by monsoon winds from the Indian Ocean and Bay of Bengal. The "wet zone" and some of the windward slopes of the central highlands receive up to of rain each year, but the leeward slopes in the east and northeast receive little rain. Most of the east, southeast, and northern parts of Sri Lanka constitute the "dry zone", which receives between of rain annually. The arid northwest and southeast coasts receive the least rain at per year. Periodic squalls occur and sometimes tropical cyclones bring overcast skies and rains to the southwest, northeast, and eastern parts of the island. Humidity is typically higher in the southwest and mountainous areas and depends on the seasonal patterns of