Te Rua Rere — sometimes anglicised as Teuruaa — is the largest known cave on Mangaia and one of the most extensive in the South Pacific. The entrance is a slit in the makatea cliff inland from Oneroa; from there the cave opens into a series of chambers with stalactites, an underground pool, and pre-Christian burial sites tucked into ledges above the floor.
Access is by guided tour only — bookings are made through one of the guesthouses, usually a day in advance. The walk in crosses sharp coral and the chambers are dim and slippery; closed shoes and a head torch help even with a guide's lamps. Allow three hours.