Corals are themselves not coral reefs—the term refers to the tiny marine invertebrates that build the rigid reef.

View the three types of coral reefs, categorized by where they form

Fringing reefs—the most common type—are built from coral depositing calcium carbonate onto the seafloor closest to the shoreline. Barrier reefs grow further from and parallel to the shore, while atolls are the result of mid-ocean volcanoes or seamounts eroding away and leaving a lagoon surrounded by reefs.

The importance of coral reefs since their first appearance hundreds of millions of years ago

Formed from calcium carbonate skeletons deposited by hard corals, some estimates suggest these reefs provide habitats for over a million marine species. Spanning massive areas, coral reefs also serve as buffers against extreme weather, such as tropical storms.

Learn about deep water corals and the coral gardens they slowly build in the dark

Found throughout the world's ocean basins at depths beyond 200 meters (660 feet), these corals do not rely on photosynthetic organisms for nutrients and instead rely on food that's fallen from the ocean surface. Beyond forming larger reefs over the course of thousands of years, smaller coral gardens are built with the aid of underwater worms.

Watch a time-lapse of hard coral growth, which forms coral reefs

Over several weeks in an aquarium, these staghorn coral polyps grew and produced new polyps as they deposited calcium carbonate, which forms the basis of coral reefs. At the 0:28 mark, you can see coral growth healing a damaged branch.

The marine animals that rely on coral reefs for habitats, camouflage, and food

Parrotfish and surgeonfish feed on algae that grow on coral reefs to prevent overgrowth while serving as prey for larger fish across the food chain. Reefs also serve as structures for organisms like sea anemones, which provide shelter for clownfish, and homes for invertebrates such as crabs, lobsters, and shrimp, which help recycle nutrients.

Coral reefs get their colors from zooxanthellae—microorganisms that line corals' digestive tracts and provide them with sugars in exchange for a home within this protective tissue.

Artificial coral reefs have been 3D printed from nontoxic, biodegradable terracotta clay, which coral polyps have used as a foundation for rebuilding marine ecosystems.

Artificial coral reefs have been 3D printed from nontoxic, biodegradable terracotta clay, which coral polyps have used as a foundation for rebuilding marine ecosystems.

Coral reefs get their colors from zooxanthellae—microorganisms that line corals' digestive tracts and provide them with sugars in exchange for a home within this protective tissue.

The anatomy of coral polyps, whose mouths consume food and expel waste

The mouth is surrounded by tentacles that help clear away debris and defend the organism, using stinging cells called nematocysts that resemble those of jellyfish. Connective tissue called coenosarc connects individual polyps within colonies and allows them to share nutrients.

Overview: All about coral reefs

Coral reefs aren’t just vibrant underwater landscapes; they are also some of the most biologically diverse ecosystems on Earth. The National Oceanic and Atmospheric Administration (NOAA) offers a comprehensive guide that explores coral reefs in...

The origin of the symbiotic relationship between corals and the algae that feed them

An analysis of fossilized reefs from northern Africa and Germany suggests that shallow water corals and photosynthetic algae formed a mutualistic partnership between 419 and 359 million years ago. The discovery was made by analyzing nitrogen-isotope ratios in the fossils, which differ depending on whether algae are present, due to various chemical processes.

How climate change threatens coral reefs, including via warming and acidification

Increasing ocean temperatures disrupt the symbiotic relationship of corals and algae, causing the former to bleach and weaken, and strengthen storms that can destroy reef structures. Higher greenhouse gas emissions increase dissolved carbon dioxide levels in ocean water, triggering a series of reactions that consume carbonate, a key ingredient in coral reef construction.

Coral polyps reproduce via broadcast spawning, where entire reefs simultaneously release reproductive cells for about one hour, once a year.

Learn how humans can protect coral reefs

When diving and snorkeling in environments containing reefs, avoid touching reefs, anchoring on them, or using sunscreen with active ingredients harmful to marine life. Properly recycling and disposing of trash in bins helps reduce microplastics and chemicals that can run off into wastewater and into the oceans.

Coral polyps reproduce via broadcast spawning, where entire reefs simultaneously release reproductive cells for about one hour, once a year.

Niagara’s bedrock holds 440-million-year-old coral fossils

The Lockport Dolomite that caps Niagara Falls is composed of fossilized coral reefs from a tropical sea that once covered the region 440 million years ago.

Synthetic pheromones may protect coral reefs by attracting their predators

Researchers in Australia and Japan have developed these nontoxic chemicals to attract crown-of-thorns starfish, which consume corals that make up the Great Barrier Reef. As of 2026, synthetic pheromones have been shown to be effective in lab trials, and future field trials will attempt to lure starfish into areas for mass removal.

Distinguishing the three types of mineral deposits collected via deep-sea mining

The greatest interest is in polymetallic nodules—structures formed from metals in seawater collecting around a core of shell, bone, or rock. Cobalt-rich ferromanganese crusts cover underwater mountain ranges and form over millions of years of metal deposition. Polymetallic sulfide deposits accumulate when seawater seeps into volcanic rocks.

'Coral Castle,' Florida's self-built monument to a lost valentine

Latvian Ed Leedskalnin moved to the US shortly after his teenage fiancée ditched him. He quickly began work building a romantic tribute to this tragic episode via massive oolitic limestone. Working mostly at night, the Latvian cultivated an air of mystique as bystanders grew curious over his methods. Today, the castle stands as a bittersweet monument to unrequited love.

Dinosaurs are defined by specific traits that many dinosaur-like creatures lack

Although pterodactyls, mosasaurs, crocodilians, and other organisms are mistaken for dinosaurs, they do not have upright legs or specific hip structures characteristic of the terrestrial reptiles. Some, like the Dimetrodon, are more closely related to humans than dinosaurs.

Speleothems are diverse mineral formations that form in air-filled caves

Speleothems like stalactites and stalagmites form when water deposits minerals after a cave rises above the water table and fills with air. These formations preserve the mineral deposits, which may serve as climate proxies, for as long as a million years.

Minerals, crystals, rocks, and gems explained

Have you ever wondered what differentiates a mineral from a rock or a crystal from a gem? This three-minute video from the American Museum of Natural History elaborates on the unique properties and compositions that separate minerals, rocks,...

Gastroliths, or ‘stomach stones,’ help animals digest plants in their stomachs

Most often used by birds and reptiles, gastroliths help break down the cellulose of plant cells, which can resist decomposition from stomach acid alone. Over time, gastroliths become smooth and are regurgitated before being replaced by larger stones.

Asteroids are the rocky remains of the solar system's formation

These celestial bodies were not integrated into the precursors of planets about 4.6 billion years ago and instead continue to orbit the sun, primarily within a band between Mars and Jupiter—the asteroid belt.

Roman concrete lasts longer than modern recipes—and even gets stronger

Today's concrete is made by binding ingredients, including chalk and silica sand, at high temperatures. Unfortunately, it weakens when exposed to salt water. This article explores how and why scientists are studying ancient Roman concrete, which grows stronger when exposed to salt water.

While geography pertains to Earth's surface features, areography pertains to Mars

Unlike Earth, Mars lacks geological activity and oceans, though geological evidence suggests they both existed in the past. Its northern regions have far fewer craters than the southern ones, indicating that the rock in the north formed more recently.

Planarian flatworms are small, aquatic worms with exceptional regenerative capabilities. When cut, the worms can regenerate multiple bodies in a matter of weeks.

Explore the history, structure, and species of Australia's Great Barrier Reef with this interactive journey narrated by Sir David Attenborough.

What is asbestos, actually?

Asbestos can hurt you, but not many people understand how or why. Asbestos is actually a stone, once frequently employed as a construction material due to its strength, softness, fire-resistance, and biodegradability. However, when working on old homes or furniture, it may be released into the air, and extended exposure to its particles may cause lung cancer. Its use in new construction has been banned, but it remains a hazard. Learn more here.

Watch how deep-sea mining equipment collects and processes polymetallic nodules—metal-rich, potato-shaped rocks—from the ocean floor and creates environmentally damaging plumes of sediments.

Volcanoes 'die' when their supply of magma runs out, if they are shifted away from their underlying magma reservoir, or if they get clogged by collapsing or their magma cooling back into rock.

Larger animals like giraffes, elephants, and blue whales have developed a unique defense against cancer, a phenomenon known as Peto's Paradox

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

SpongeBob exemplifies Aristotle's philosophy for living well because he understands that happiness doesn't come from external factors like money, recognition, or power.

Some Pokémon have been named after real and fictional organisms, while some real-world species have been named after Pokémon, such as Chilicola charizard and Binburrum zapdos.

The animatronic shark from Steven Spielberg's 'Jaws,' the first modern blockbuster, was a complex underwater apparatus called 'Bruce.'

Shooting stars, or meteors, are fragments of asteroids and comets that burn up as they enter Earth's atmosphere, creating streaks in the sky called meteor showers.

All rainbows are 'raincircles' caused by water droplets separating white sunlight into a spectrum of colors, but Earth's surface blocks the bottom half of the circle from our line of sight.

Fear or disgust related to clusters of holes and cracks, called trypophobia, may stem from evolutionary processes to help us avoid pathogens.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

Some coin denominations have ridges etched on their sides, partly to prevent counterfeiting, in a practice called 'reeding.'

The term 'algorithm' originated from the Latinized name of al-Khwārizmī, a 9th-century mathematician whose works introduced Arabic numerals and the decimal-based numeral system to Europe and whose treatise 'Al-Jabr' gave algebra its name.

Ebola is a filovirus, meaning it resembles a thread under a microscope, though it sometimes twists and can resemble a bowl of spaghetti.

Poison ivy's rash-causing compound is found in the plant's roots, stem, and leaves—so coming into contact with any part of the plant can lead to rashes for the 85% of people who are allergic to it.

Brian Wilson wrote the Beach Boys' 'Don't Worry Baby' for the Ronettes, but the legendary girl group rejected it.

Iran's Revolutionary Guard Corps is independent of Iran's regular army and is one of the most powerful and influential military-political organizations in the Middle East.

Hotels in Cannes, the glamorous city on the French Riviera, hire hawks to ward off seagulls during the city's famous film festival.

Gerrymandering works through two techniques: cracking opposing voters across districts to dilute their power, and packing them into a few to waste their votes in lopsided majorities.

'Grand Canyon of the Yellowstone,' a work by American painter Thomas Moran, inspired Congress to establish Yellowstone National Park.