Coral grows on every continent’s coastline in tropical and subtropical waters, spanning a belt roughly between 35°N and 35°S latitude. But coral isn’t limited to warm, shallow reefs. Deep-sea species live in cold, dark waters thousands of feet below the surface, and reef systems range from massive structures visible from space to small patches hugging rocky coastlines.
The Tropical and Subtropical Belt
Most reef-building corals cluster in warm, sunlit waters where temperatures stay between 23°C and 29°C (roughly 73°F to 84°F). This restricts the bulk of the world’s reefs to a band circling the globe near the equator, from the southern tip of Japan and the Florida Keys in the north down to southern Brazil and Madagascar in the south. Some corals tolerate brief spikes up to 40°C (104°F), but sustained heat causes bleaching and die-off.
Light is the other non-negotiable requirement. Reef-building corals depend on tiny algae living inside their tissues that convert sunlight into energy. That relationship means most reefs grow in shallow, clear water where sunlight can penetrate. Murky or sediment-heavy water blocks light and limits how deep reefs can develop. Salinity matters too: corals generally need ocean water in the range of about 29 to 40 parts per thousand, which rules out areas near large river mouths where freshwater dilutes the sea.
The Coral Triangle: Earth’s Richest Reef Region
If you’re looking for the greatest diversity of coral on the planet, it’s in the Coral Triangle, a roughly triangular stretch of ocean spanning parts of Indonesia, Malaysia, Papua New Guinea, the Philippines, the Solomon Islands, and Timor-Leste. This region contains 605 of the world’s roughly 798 known coral species, about 76% of all reef-building corals. No other marine region comes close to that concentration.
The Coral Triangle sits at the crossroads of the Indian and Pacific Oceans, where warm currents, complex coastlines, and thousands of islands create an enormous range of habitats. Reefs here range from fringing reefs along volcanic islands to massive atolls in open ocean. For snorkelers and divers, destinations like Raja Ampat in Indonesia and Tubbataha Reef in the Philippines are considered some of the most biodiverse underwater environments anywhere.
The World’s Largest Reef Systems
Australia’s Great Barrier Reef is the largest coral reef system on Earth, stretching 1,429 miles along the coast of Queensland in the Coral Sea and covering approximately 133,000 square miles. It’s actually a mosaic of about 2,900 individual reefs and 900 islands rather than a single continuous structure.
Other major systems include the Mesoamerican Barrier Reef, which runs along the Caribbean coast from Mexico’s Yucatán Peninsula through Belize, Guatemala, and Honduras. It’s the largest reef system in the Atlantic. The Red Sea hosts some of the most productive and diverse reefs outside the Indo-Pacific, with a notable quirk: its northern reefs appear more resistant to warming and ocean acidification than reefs elsewhere, making them a potential refuge for coral species in the coming decades. The New Caledonia Barrier Reef in the South Pacific is the second longest continuous reef after Australia’s.
Coral Reefs in the United States
The U.S. has more reef territory than many people realize, spread across several states and territories. More than 60% of American coral reefs sit in the extended Hawaiian Island chain, which stretches over 1,500 miles across the north-central Pacific. The Northwestern Hawaiian Islands alone, protected within the Papahānaumokuākea Marine National Monument, contain vast reef systems largely untouched by coastal development. The main Hawaiian Islands add over 140,000 acres of reef habitat on top of that.
Florida’s Reef Tract is the third largest barrier reef ecosystem in the world. It runs from St. Lucie Inlet in Martin County, north of Miami, all the way to the Dry Tortugas west of the Florida Keys. About two-thirds of this reef falls within Biscayne National Park and the Florida Keys National Marine Sanctuary.
Beyond those two well-known spots, the U.S. has significant reefs off Puerto Rico and the U.S. Virgin Islands in the Caribbean, as well as around American Samoa, Guam, and the Commonwealth of the Northern Mariana Islands in the Pacific. Guam and American Samoa in particular have fringing reefs that support both local fisheries and tourism.
Deep-Sea Coral: Cold and Dark
Not all coral needs warm water or sunlight. Deep-sea corals (sometimes called cold-water corals) grow on the ocean floor at depths that can exceed 600 meters, roughly 2,000 feet. Unlike their tropical relatives, these corals don’t rely on photosynthetic algae. They feed by filtering tiny organisms and particles from the water column, which means they can survive in complete darkness.
Deep-sea corals have been documented in the Gulf of Mexico, the North Atlantic off Norway and the British Isles, the Aleutian Islands near Alaska, and along submarine canyons off the U.S. East Coast. Norway’s Røst Reef, a cold-water coral structure, is one of the largest known examples. These reefs grow extremely slowly compared to tropical corals, sometimes adding less than a millimeter per year, which makes them especially vulnerable to damage from bottom trawling and deep-sea mining.
While you won’t see deep-sea coral on a vacation snorkel trip, these ecosystems are ecologically significant. They serve as nurseries and shelter for fish and invertebrates in otherwise barren stretches of ocean floor.
Less Obvious Places Coral Shows Up
A few coral habitats surprise people. The eastern Pacific coast of Central and South America has scattered reef patches despite cold upwelling currents that generally discourage reef growth. Japan’s Ryukyu Islands, stretching southwest from the mainland toward Taiwan, host extensive subtropical reefs that mark one of the northernmost limits of reef development in the Pacific. Bermuda, sitting at about 32°N latitude, is often cited as the most northerly coral reef system in the Atlantic, kept viable by the warm Gulf Stream.
In the Indian Ocean, coral reefs fringe much of the Maldives, the Seychelles, and the coast of East Africa from Kenya down through Tanzania and Mozambique. The Maldives is essentially built on coral: the nation’s 1,200 islands are the tops of ancient coral atolls sitting on a submerged volcanic ridge. Without living reef to protect those low-lying islands from wave erosion, they would gradually disappear.
Even temperate waters along southern Australia and the Mediterranean Sea support non-reef-building coral species. These corals grow as individual colonies or small clusters rather than constructing the massive limestone frameworks associated with tropical reefs, but they are genuine corals with the same basic biology.