Mangaia is the second-largest island of the Cooks (51,8 sq km), with a population of 780. Its geography is basically similar to Atiu, but it is much more dramatic. The makatea rises rapidly from the coast and, in most places, it drops as a sheer wall to the inner depression. The water from the central depression sinks at the inner wall of the makatea and emerges in several small springs along the coastline. After heavy rains, streaks of Mangaia's red soil can be seen stretching from the makatea into the sea. The makatea is riddled with smaller caves.
Text from George Szentes (2004): Caves of the Cook Islands, The British Caver Vol. 127 Spring 2005, pp 1-12, 13 color pictures, 1 bw picture, 1 map. With kind permission of the author.
The caves are owned by families and located on private land. Some caves are open for guided tours by family members. There are no open hours, as far as we understand you drive nearby and ask someone at the road who will direct you to the home of the owner. The island has only a few hundred inhabitants and everybody knows everybody. Probably the owners will know about you before you get there.