A ring-shaped coral reef or chain of low islands surrounding a lagoon. Most atolls formed as reefs kept growing upwards around volcanic islands that slowly sank beneath the sea.

Why is tropical sand so white?

Because it is made mostly of fragments of coral, shells and algae, which are pale calcium carbonate. Parrotfish produce a lot of it by grinding up coral as they feed.

Is coral a plant or an animal?

Coral is an animal. Each colony is made of many tiny polyps, but they live in partnership with microscopic algae that make food from sunlight.

What causes coral bleaching?

Mainly water that stays too warm for too long. Stressed corals expel their partner algae and turn white. They can recover if conditions improve, but long or repeated bleaching can kill them.

Can I explore a tropical beach and reef in the app?

Yes. You can land on a South Pacific island in interactive 3D, walk the beach and then swim and dive onto the coral reef.

White sand, turquoise lagoons and the living cities of the coral reef

Tropical beach & reef

3D landscape · Moʻorea, French Polynesia

Land here and explore →

Coral reefs grow in warm, clear, shallow seas, mostly within about 30 degrees of the equator. They are built by colonies of small animals called coral polyps, which lay down skeletons of limestone over centuries and millennia. Around volcanic high islands and on the rims of atolls, these reefs create calm lagoons and beaches of brilliant white sand. In this app you can land on a South Pacific island modelled on an atoll and high island, then swim and dive onto the reef in interactive 3D.

Reefs cover only a tiny fraction of the sea floor, yet they are among the richest ecosystems on Earth, home to a huge variety of fish, invertebrates and algae. They protect coastlines from waves, support fisheries and tourism, and help sustain hundreds of millions of people. They are also highly sensitive to warming seas, and marine heatwaves have caused widespread coral bleaching in recent years.

Key facts

Model setting: South Pacific atoll and high islandA volcanic island ringed by reef, with low coral islets nearby
Reef builders: Coral polypsSmall animals related to jellyfish and sea anemones
Partner algae: ZooxanthellaeMicroscopic algae living inside coral tissue
Preferred water: Roughly 23–29°CMost reef-building corals need warm, clear, shallow water
Share of sea floor: Less than 1%Yet reefs support an estimated quarter of known marine species
Largest reef system: Great Barrier ReefStretching over 2,000 km along north-eastern Australia
Atoll theory: Charles Darwin, 1842Confirmed by deep drilling at Enewetak Atoll in the 1950s
White sand source: Coral and shell fragmentsMuch of it ground up and excreted by parrotfish
Global bleaching: Fourth event declared in 2024Announced by NOAA and the International Coral Reef Initiative

How atolls form

During the voyage of HMS Beagle, Charles Darwin puzzled over atolls: rings of coral islands surrounding a lagoon, often far from any other land. In 1842 he proposed that they begin as fringing reefs growing along the shore of a volcanic island. As the extinct volcano slowly sinks and erodes, the corals keep growing upwards towards the light. The reef becomes a barrier reef, separated from the island by a lagoon, and when the volcano finally disappears beneath the sea, only the ring-shaped atoll remains.

Darwin's subsidence theory was confirmed more than a century later. In the early 1950s, drilling at Enewetak Atoll in the Marshall Islands passed through well over a kilometre of reef limestone before reaching volcanic rock. The South Pacific shows every stage of the process: high islands such as Bora Bora and Moorea with their surrounding reefs, and atolls such as those of the Tuamotus and Kiribati, where the highest ground may be only a few metres above sea level.

Where white sand comes from

Most beach sand in the world is made of quartz eroded from continental rock, but tropical coral beaches are different. Their bright white sand is largely biological, made of broken pieces of coral skeleton, shells, sea urchin spines and the limestone plates of green algae such as Halimeda. Waves grind these fragments down over time, and because they are made of calcium carbonate, the sand is pale and stays cool underfoot compared with dark volcanic sand.

Parrotfish play a surprising part. They scrape algae from dead coral with their beak-like teeth, swallowing bits of limestone as they go, and grind it into fine sediment that passes through their gut. A large parrotfish can produce a great deal of sand in a year; estimates run to hundreds of kilograms for some species. On some reefs, scientists think parrotfish produce a large share of the sand that builds beaches and islands.

Lagoons, reef flats and islands

Behind the outer reef lies the lagoon, a sheltered body of water whose famous turquoise colour comes from sunlight reflecting off a pale sandy floor through clear, shallow water. Lagoons can be dotted with patch reefs and coral heads, and they serve as nurseries for young fish, rays and sharks. Passes through the reef let water, nutrients and animals flow between the lagoon and the open ocean with the tides.

The low islands on an atoll's rim, often called motu in Polynesia, are made of sand and coral rubble piled up by waves and storms. They are naturally dynamic, shifting shape over years and decades. Their freshwater comes mainly from rain, which can float as a thin lens on top of the denser seawater in the ground. This makes atoll communities especially vulnerable to drought, storms and rising sea levels.

Coral biology and symbiosis

A coral colony is made of many polyps, each a small soft-bodied animal with a ring of stinging tentacles around its mouth. Polyps catch plankton at night, but most of their food comes from partner algae, known as zooxanthellae, which live inside their tissues. The algae use sunlight to make sugars and share much of this energy with the coral, while the coral provides shelter and nutrients. The algae also give corals much of their colour.

Fuelled by this partnership, reef-building corals deposit calcium carbonate skeletons beneath their living tissue. Branching corals can grow several centimetres a year, while massive boulder corals grow far more slowly, perhaps a centimetre or so. Many corals reproduce in spectacular mass spawning events, releasing clouds of eggs and sperm on a few nights each year, timed by water temperature and the phase of the Moon.

Reef types

Scientists usually describe three main reef types: fringing reefs that grow close to shore, barrier reefs separated from the coast by a lagoon, and atolls that encircle a lagoon with no central island.

Reef biodiversity

Coral reefs are often called the rainforests of the sea. The nooks and crannies of the reef provide food and shelter for an enormous range of life: fish such as parrotfish, wrasses, butterflyfish, groupers and reef sharks; invertebrates such as giant clams, sea stars, octopuses, sea urchins and crustaceans; and sea turtles, which graze and hunt on the reef. Much of reef diversity is still undescribed, especially among small invertebrates.

Many relationships on the reef are partnerships. Clownfish shelter among the stinging tentacles of sea anemones, cleaner wrasses remove parasites from larger fish at cleaning stations, and gobies share burrows with shrimps. Grazing fish and urchins keep seaweed in check, giving young corals space to settle. When these grazers are overfished, algae can smother a reef.

Bleaching and other threats

When water stays unusually warm for weeks, the partnership between coral and algae breaks down. The coral expels its zooxanthellae and turns white, revealing its skeleton through clear tissue. This is coral bleaching. Bleached corals are not dead, and they can recover if temperatures fall soon enough, but prolonged or repeated bleaching can kill them. In 2024 NOAA and the International Coral Reef Initiative confirmed a fourth global bleaching event, affecting reefs across all major ocean basins.

Reefs also face ocean acidification, as seawater absorbs carbon dioxide and it becomes harder for corals to build skeletons. Pollution, sediment runoff, overfishing, destructive fishing, disease and outbreaks of coral-eating crown-of-thorns starfish add further stress. Protecting reefs involves cutting greenhouse gas emissions, managing fishing and water quality, and, increasingly, restoration projects that grow and replant heat-tolerant corals.

Surprising facts about Tropical beach & reef

Much of the white sand on tropical beaches has passed through the gut of a parrotfish.
Corals are animals, but most of their food is made by microscopic algae living inside them.
Charles Darwin worked out how atolls form before he published his theory of evolution.
Drilling at Enewetak Atoll went through more than a kilometre of coral limestone before reaching the volcano beneath.
Coral reefs cover less than 1% of the sea floor but support roughly a quarter of known marine species.
Many corals spawn on just a few nights a year, timed by the Moon, filling the water with clouds of eggs.

Frequently asked questions

What is an atoll?: A ring-shaped coral reef or chain of low islands surrounding a lagoon. Most atolls formed as reefs kept growing upwards around volcanic islands that slowly sank beneath the sea.
Why is tropical sand so white?: Because it is made mostly of fragments of coral, shells and algae, which are pale calcium carbonate. Parrotfish produce a lot of it by grinding up coral as they feed.
Is coral a plant or an animal?: Coral is an animal. Each colony is made of many tiny polyps, but they live in partnership with microscopic algae that make food from sunlight.
What causes coral bleaching?: Mainly water that stays too warm for too long. Stressed corals expel their partner algae and turn white. They can recover if conditions improve, but long or repeated bleaching can kill them.
Can I explore a tropical beach and reef in the app?: Yes. You can land on a South Pacific island in interactive 3D, walk the beach and then swim and dive onto the coral reef.

Keep exploring

The landscapes in Ember’s Universe are procedural 3D scenes modelled on real places. The landforms, plants and light follow the real thing, but the details are generated, not mapped.

Sources