The island is surrounded by a coral reef, which is exposed at low tide and dotted with pinnacles. The reef is bounded seaward by deep water, and on the inside by a sandy beach. The presence of the reef has prevented the establishment of a seaport, although 16 channels in reef allow small boats access to the island. A 150 to 300 metre (about 500 to 1,000 ft.) wide fertile coastal strip lies inland from the beach.

Coral cliffs surround Nauru's central plateau, which is known as "Topside". The highest point of the plateau, called the Command Ridge, is 71 metres above sea level. The only fertile areas on Nauru are the narrow coastal belt, where coconut palms flourish. The land surrounding Buada Lagoon supports bananas, pineapples, vegetables, pandanus trees, and indigenous hardwoods such as the tomano tree. The population of Nauru is concentrated in the coastal belt and around Buada Lagoon.

Nauru was one of three great phosphate rock islands in the Pacific Ocean (the others were Banaba (Ocean Island) in Kiribati and Makatea in French Polynesia). However, the phosphate reserves on Nauru are depleted for all practical purposes. Phosphate mining in the central plateau has left a barren terrain of jagged limestone pinnacles up to 15 metres (49 ft) high. A century of mining has stripped and devastated about 80% of the land area. Mining has also affected the surrounding Exclusive Economic Zone, with 40% of marine life estimated to have been killed by silt and phosphate runoff.