Mataiva Atoll is a single island in the Tuamotu Archipelago, a small part of the longest chain of atolls in the world. An atoll is just a ring-shaped island, a sandy reef with a central lagoon, but Mataiva is a little bit strange. Mataiva's lagoon has internal structure.
Along with that whole evolution-thing for animals, Charles Darwin also figured out the sequence of island change over time that resulted in the formation of atolls. A volcano builds up, growing in eruption after eruption until finally breaking the waves and emerging as an island. If it is in warm, tropical waters, a reef may start to grow, circling its flanks just below the water line. The island may continue growing for some time, but eventually erosion will win, and it will begin to sink back below the waves. The reef will grow to keep pace with the water's surface, growing straight up as the volcano sinks until a lagoon grows between reef and land. This fringing reef keeps growing as the island sinks, until at last the final peak disappears below the waves, and only a ring remains. This is an atoll, one of the most picturesque of island types.
The 10-kilometer island of Mataiva has the central lagoon marking it as an atoll, but the lagoon is crossed with a web of ridges separating small basins. At no stage in the nice, simple geological evolution of an atoll does the central lagoon generate a network of ridges. The structures are composed of broken and eroded reefs, rocky arms dividing the lagoon into basins. As the reefs erode further, the broken pieces form the iconic white sands that make tropical beaches so picturesque.
The name Mataiva translates into "nine eyes" in the local Tuamotuan language. The nine eyes are the nine channels connecting the pale blue shallow waters of the lagoon to the dark blue deep waters of the Pacific Ocean. Another, wider channel crosses the atoll in the north, marking Pahua, the island's only village. The local economy is focused on exporting vanilla and dried coconut (copra), although tourism is of growing importance since an airport was constructed in 1999.
Image credit: NASA. Read more on the Earth Observatory. For another look at delicate marine features, check out Florida or the Bahamas. Poke around the discussion for other examples of infilled atolls, including Christmas Island.