Atolls of The Leeward islands begin as volcanic islands (see Islands Map Trek). In the tropics, coral reefs grow just offshore (see Coral Reefs Map Trek). Eventually the volcanoes that formed the islands become extinct. No longer pushed upward by hot, molten rock, the sea floor beneath the islands begins to sink. Meanwhile, gradual erosion wears the islands down. The islands of Raiatea and Tahaa in French Polynesia once formed a single volcanic landmass.
Today these two islands, surrounded by a coral reef, are all that remain above the surface. To the north of the Leeward islands lies Bora-Bora, another reef-fringed sunken volcano. Eventually, the eroded volcano sinks completely beneath the sea, but the coral reef grows upward as fast as the volcano sinks. What remains is a lagoon and the fringing reef of an atoll, such as Tupai, north of Bora-Bora.