Located in Oceania, Tonga is an archipelago in the South Pacific Ocean, directly south of Western Samoa and about two-thirds of the way from Hawaii to New Zealand. Its 176 islands, 36 of them inhabited, are divided into three main groups – Vava'u, Ha'apai, and Tongatapu – and cover an 800-kilometre (500-mile)-long north-south line. The largest island, Tongatapu, on which the capital city of Nukuʻalofa is located, covers 257 square kilometres (99 sq mi). Geologically the Tongan islands are of two types: most have a limestone base formed from uplifted coral formations; others consist of limestone overlaying a volcanic base.