The ancient lagoon of the Makatea elevated atoll is a ruiniform coral limestone plateau. Widely eroded, dissolved by rainwater and the action of the ocean, excavated by the phosphate exploitation, it has a surface of 17.3sqmi (28km2) where caves, tunnels and cavities abund. The feo of the ancient deposits returning the solar energy stored during the day, the underwater explorations avoid the stifling atmosphere of aerial cavities.