The Pitcairn Islands form the southeasternmost extension of the geologicalarchipelago of the Tuamotus of French Polynesia, and consist of four islands: Pitcairn Island, Oeno Island (atoll with five islets, one of which is Sandy Island), Henderson Island and Ducie Island (atoll with four islets). The only permanently inhabited island, Pitcairn, is accessible only by boat through Bounty Bay. Henderson Island, covering about 86% of the territory's total land area and supporting a rich variety of animals in its nearly inaccessible interior, is also capable of supporting a small human population, but access is difficult due to its outer shores being steep limestone cliffs made of sharp coral. The Pitcairn Islands were formed by a centre of upwelling magma called the Pitcairn hotspot. The other islands are at a distance of more than 100 km (60 mi) and are not habitable.