Nauru, one of the largest phosphate-rock islands in the Pacific, is oval-shaped and fringed by a wide coral reef. It has no natural harbor or anchorage. A relatively fertile belt varying in width from 150 to 300 m (490–980 ft) encircles the island. From this belt a coral cliff rises to a central plateau about 60 m (200 ft) above sea level. Buada Lagoon, a permanent, often brackish lake, covers some 300 acres (1.2 km/0.47 sq mi) in the southeastern end of the plateau. Apart from some brackish ponds and an underground lake, the nation's water supply is provided by rainfall.