Tuvalu consists entirely of low-lying coral atolls, none of which is more than 5 m (16 ft) above sea level; few of the atolls are more than 0.8 km (0.5 mi) wide. The islands are coral reefs on the outer arc of ridges formed by pressure from the Central Pacific against the ancient Australian landmass. On five islands, the reefs enclose sizable lagoons; the others are mere pinnacles rising abruptly from the ocean floor. Only two of the islands, Funafuti and Nukufetau, have natural harbors for oceangoing ships. There are no rivers on the islands.