An atoll is a ring-shaped coral reef, island, or series of islets. An atoll surrounds a body of water called a lagoon. Sometimes, atolls and lagoons protect a central island. Channels between islets connect a lagoon to the open ocean or sea.

Atolls develop with underwater volcanoes, called seamounts. First, the volcano erupts, piling up lava on the seafloor. As the volcano continues to erupt, the seamount's elevation grows higher, eventually breaking the surface of the water. The top of the volcano becomes an oceanic island.

In the next stage, tiny sea animals called corals begin to build a reef around the island. The type of corals that build reefs are called hermatypic corals, or hard corals. Hermatypic corals create a hard exoskeleton of limestone (calcium carbonate). Billions of these limestone exoskeletons are the reef.

This coral reef, called a fringing reef, surrounds the island just below the ocean surface. The thin, shallow strip of water between the fringing reef and the island is the lagoon.

Over millions of years, the volcanic island erodes and sinks to the seafloor. This process is called subsidence. The seamount erodes into the sea, its top made flat by the constant pounding of powerful ocean waves. As it subsides, the flat-topped seamount is called a guyot.

As the island subsides to become a guyot, its ring-shaped fringing reef turns into a barrier reef. A barrier reef is farther from shore, and has a deeper lagoon. The barrier reef protects the lagoon from the harsh winds and waves of the open ocean.

Subsidence brings slight differences in ocean chemistry that change the reef radically. The outer, ocean-facing side of the reef remains a healthy marine ecosystem. Corals on the inner, lagoon-facing side, however, begin to slowly decay. The algae that corals need to survive face much more competition for fewer nutrient resources. The limestone decays, changing the color of the lagoon from deep ocean blue to bright teal.

In the final stage of an atoll’s formation, ocean waves break apart pieces of the limestone reef. They pound, break, and erode the coral into tiny grains of sand. This sand and other material deposited by waves or wind pile up on the reef. This material, including organic matter such as plant seeds, form a ring-shaped island or islets. This is an atoll.

Hermatypic corals only live in warm water. An island that is located where ocean temperatures are just warm enough to support hermatypic corals is said to be at the “Darwin point," named after Charles Darwin. The famous naturalist was the first to outline how atolls form.

Atolls and People

The rocky or sandy shores of atolls have been important sites throughout human history. Often, their low-lying elevation has proved perilous.

Atolls are often hidden by ocean waves. Thousands of ships, from ancient Polynesian canoes to sophisticated American warships, have been stranded and wrecked on hidden atolls.

The Kon-Tiki, probably the most famous raft in history, became one of these atoll casualties. The Kon-Tiki was a large balsa raft built and sailed by explorer Thor Heyerdahl and his crew in 1947. The Kon-Tiki successfully sailed 6,980 kilometers (4,340 miles) from Peru to the South Pacific. The most difficult challenge of the journey was not the waves, currents, or trade winds of the open ocean. It was the atolls of Polynesia, the final part of their journey.

The quick-moving currents around atolls prevented the Kon-Tiki from docking at the first Polynesian island it encountered. It wrecked on the shallow coral of the second, Raroia atoll. Raroia was uninhabited, but nearby Native islanders in canoes eventually rescued the Europeans on the washed-up wreck.

The Kon-Tiki was eventually hauled out of Raroia, but atoll wrecks are popular dive sites throughout the Pacific. Shipwrecks from the 18th century through World War II lie at the bottom of atolls such as Kwajalein, part of the Marshall Islands.

Atolls are often uninhabited "desert" islands. (Desert does not refer to the islands' climate, but their "deserted" or uninhabited status.) Many are remote and difficult to reach. In the 20th century, this isolation made them attractive as testing sites for nuclear weapons from the United States, Britain, and France.

The first hydrogen bomb, for instance, was tested by the United States at Bikini Atoll, part of the Marshall Islands in the Pacific Ocean. The Pacific Proving Grounds, a series of 2,000 atolls and other islands under U.S. jurisdiction, were the site to more than a hundred massive nuclear explosions between 1947 and 1962. France continued nuclear testing on the atoll of Moruroa until 1995.

Nations throughout Polynesia, including the "nuclear-free zone" of New Zealand, protested extended nuclear testing. Reefs were being destroyed, and some tests dropped toxic fallout onto nearby inhabited islands. After Castle Bravo, the first hydrogen bomb test, the U.S. evacuated residents of the Rongelap and Rongerik atolls, and later compensated them for medical conditions associated with radiation poisoning.

The same elements that make atolls popular for nuclear testing also make them attractive to tourists. Atolls are sparsely populated, low-lying islands whose white, sandy beaches and placid lagoons are ideally suited to the tourism industry.

Island nations made of atolls include Maldives, in the Indian Ocean, and Kiribati, in the Pacific. Tourism is a key factor in both the Maldivian and Kiribati economies.

Many atolls, however, have few tourists and are among the world's underdeveloped countries. Atolls have few natural resources. Soil quality on atolls is very poor, and erosion is a constant threat. Most native residents on atolls practice subsistence agriculture and fishing. Almost all food and fuel is imported, often at great cost.

Fisheries and support for the shipping industry help support communities on remote atolls. Some atoll communities have taken advantage of their equatorial location and established launch sites for low-orbit satellites. Others have found other sources of revenue. The nation of Tuvalu, for instance, is a series of isolated atolls in the Pacific. Every year, it receives millions of dollars for use of its ".tv" Internet domain name.

Atolls, along with sandbars, are among islands with the lowest elevation. They are constantly, naturally at risk from erosion due to wind and waves. Atolls are also at risk from sea-level rise. As the the ocean level rises, atolls—and any infrastructure on them—are flooded and may drown altogether.

Island nations such as Maldives and Kiribati are fortifying their atolls by dredging the seafloor. Sand elevates certain areas and widens others, creating a more stable landmass.

Maldives and Kiribati have also taken political measures to protect their citizens from the possibility of atolls sinking beneath the sea. Maldives often leads international conferences on the impacts of global warming, which is associated with sea level rise. Maldives and Kiribati have also taken steps to outline a permanent evacuation process should sea level rise threaten to drown inhabited atolls.