The three types of coral reefs—fringing, barrier, and atoll—are categorized by where they form.

The first coral reefs formed hundreds of millions of years ago from the carbonate skeletons deposited by hard corals.

Deep water corals—located throughout the world's ocean basins at depths beyond 200 meters (660 feet)—slowly build small coral gardens in the dark with the help of underwater worms.

Stony corals form a type of calcium carbonate called aragonite from ions brought to the ocean by rivers that have broken down terrestrial rocks.

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...

Over 1 million fish, marine mammals, and invertebrate species depend on coral reefs for habitats, camouflage, and food.

Through coral IVF, researchers have collected coral eggs and sperm from healthy reefs, grown baby corals in lab pools, and then released them to help repopulate damaged or lost reefs.

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 are built by thousands of coral polyps—tiny animals whose mouths both consume food and expel waste.

About 76% of global coral species are found in the Coral Triangle—a region spanning the waters of Indonesia, Malaysia, Papua New Guinea, the Philippines, the Solomon Islands, and Timor-Leste.

Why are coral reefs important?

Coral reefs are vital ecosystems that millions of people and countless species depend on. This two-minute video from London's Natural History Museum looks at the ecological and economic significance of coral reefs, including their biodiversity and...

Explore a map of coral reefs around the world and the extent of coral bleaching.

Watch a time-lapse of staghorn coral polyps—a type of hard coral—over several weeks in an aquarium, slowly forming the foundational component of a coral reef.

Most volcanic activity occurs in submarine volcanoes on the seafloor

At divergent plate boundaries in mid-ocean ridges, volcanic activity produces new ocean crust as the seafloor spreads. These areas are home to underwater mountains with extensive biodiversity due to redirecting ocean currents and concentrating food for marine life.

Explore case studies of each type of convergent plate boundary

At these boundaries, a collision of two oceanic plates produces a curved chain of volcanic islands, like those seen in the Caribbean. A continental-continental plate collision results in plates buckling upward, forming large mountain ranges.

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.

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.

The National Oceanographic and Atmospheric Administration estimates that US commercial and recreational fisheries valued at $200M depend on coral reefs, which provide up to $1.8B in flood protection by absorbing 97% of wave energy.

An analysis of fossilized reefs from northern Africa and Germany suggests that shallow water corals and photosynthetic algae formed the mutualistic partnership seen today between 419 and 359 million years ago.

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

Climate change threatens coral reefs by disrupting the symbiotic relationship between corals and algae due to rising ocean temperatures.

Explore coral reefs in Google's "street view"

Expanding its mission to map the world, Google Maps partnered with The Catlin Seaview Survey to introduce the first-ever underwater panoramas, making oceanic marvels like the Great Barrier Reef accessible to everyone. With Google Maps, the public...

The world's largest coral, as seen from space

The world’s largest individual coral colony—measuring over 100 feet wide and 100 feet long—was discovered in the Solomon Islands. This coral is so large it can be seen from space, and is estimated to be between 300 and 500 years old. The coral's size and health provide hope amidst a global coral bleaching crisis, wherein three-quarters of coral reefs have experienced significant bleaching in recent decades. See the photos from space and learn more here.

When diving and snorkeling in environments containing reefs, avoid touching reefs, anchoring on them, or using sunscreen with active ingredients harmful to marine life.

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.

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.

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

Decommissioned oil rigs have been repurposed into artificial reefs that serve as nursery habitats for fish, corals, sponges, and other invertebrates in the decades since their installation.

View a gallery of diagrams for each volcano type with glossaries

The three major types of volcanoes—cinder cone, composite, and shield—each exhibit unique, lesser-known subterranean and surface features, including lava bombs, lahars, and spatter. Other types of volcanoes include lava domes, calderas, fissure volcanoes, and Maars and tuff rings.

View a collection of glass sculptures of corals threatened by fishing and deep-sea mining.

How Oceans were formed

Nonprofit organization American Oceans briefly explains the formation of the oceans, which cover over two thirds of Earth’s surface. They were formed by centuries of rain from the condensation of Earth's original gasses.

38 thought-provoking facts about the ocean

Coral produces its own sunscreen, tsunamis move at 500 miles per hour, one of the loudest sounds ever recorded came from an ice quake, and other interesting facts collected from sources such as National Geographic and NASA.

Fossils form in environments where dead organisms can be rapidly buried

Such landscapes enable corpses to be encased by terrain before significant decay occurs. They are most commonly found in river deltas and sinking coastal regions, where sediments accumulate over time until geological forces expose the areas to erosion, revealing embedded fossils.

Multiple species exhibit convergent evolution by developing crab-like features

Various organisms that appear to be different types of crabs, such as porcelain crabs and coconut crabs, have been shown to belong to distinct decapod lineages whose predecessors resemble shrimp or lobsters. Environmental pressures and other factors have caused these species to develop similar characteristics, which nature has shown are beneficial to survival.

A simple pictorial explanation of ocean geography

Visualizing ocean geographical features, such as a coastal zone, continental shelf and abyssal plain. This article from the US Geological Survey also provides several links to diverse ocean facts.

Climate tipping points are irreversible changes to Earth's environment

Earth's natural systems have prevented these events—including the mass death of coral reefs, loss of the Amazon rainforest, and shutdown of Atlantic currents—but scientists believe they are unlikely to withstand continued stressors from climate change.

Offshore wind turbines are built like giant modular puzzles at sea

Due to their enormous size, turbines are assembled in parts on-site using prefab components, advanced cranes, and vibration-dampening systems to manage the dynamic ocean environment. Three-blade turbines became the norm because even-numbered blades create damaging vibrations.

The discovery of the Mid-Atlantic Ridge was initially dismissed as ‘girl talk’

Marie Tharp identified the structure when mapping the ocean floor using sonar data, but her colleague, who did not favor continental drift, was unconvinced. Famed filmmaker Jacques Cousteau even tried to disprove her work with underwater footage, only to validate her maps.

Tropical cyclones consist of three main parts: the eye, the eyewall, and rainbands

The eye is a calm, clear central area surrounded by an eyewall of intense thunderstorms. Rainbands are spiral bands of clouds and thunderstorms extending from the eyewall.

Ring galaxies, the rarest kind in the universe

Galaxies come in different sizes, but when it comes to shapes there are just four kinds: spiral, elliptical, irregular, and the rarest kind, ring. Ring galaxies only form in the event of a small galaxy passing through a large one, but there's more to them that that. This page explores these rare and beautiful heavenly bodies.

'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.

The four types of seismic waves are categorized as body or surface waves

Primary (P) and secondary (S) waves are compression and shear waves, respectively, and travel through Earth’s interior, though secondary waves only move through solids. These waves’ energy is converted into rolling Rayleigh or horizontal Love waves at the surface.

Unique species evolved independently on each Galápagos island

The islands formed over millions of years because of a hot spot in the Nazca Plate, where magma breaks through and forms volcanoes. As these islands drifted, plants and animals became secluded, making each island its own an isolated experiment that would influence Charles Darwin's theory of evolution.

View the various types of lightning

The most common form of lightning involves channels of electricity between oppositely charged regions within a cloud or between two clouds to reestablish electrostatic equilibrium. While cloud-to-ground lightning is understood best, research is ongoing to fully model the mechanisms behind rarer forms of lightning, such as transient luminous events.

A V-shaped recovery from a recession is the most common kind

When charted, recession recoveries resemble letters including V, U, W, L, and K. For instance, an L-shaped recovery involves a steep decline that slowly reverses.

A list of common questions and answers about hurricanes from NOAA

This FAQ answers many questions hurricane researchers at NOAA have received, sorted by category. The answers provide insights into hurricane classification and naming, forecasting their movement and growth, and tips for preparing ahead of landfall.

Track individual sharks around the world in real time

OCEARCH provides essential data on shark populations and marine ecosystems to support broader conservation efforts. Foster your understanding of marine ecosystems by exploring different species and their movement.

Hot, semiarid, coastal, cold: An overview of the four desert types

This article describes the various characteristics of deserts, including their geography, their climate, their soil, and the wildlife and vegetation that inhabit them. All desert climates are characterized by extreme temperatures and low rainfall,...

Surfer's ear, from repeated cold water exposure, leads to new bony protrusions in the ear

Frequent exposure to icy cold water, typical in surfing, can cause tiny bony growths to develop in the ear canal, informally called surfer's ear. While the exact cause for these growths is unknown, it may be the body's protective response to shield the ear drum from the cold. These protrusions can heighten the risk of swimmer's ear, lead to wax buildup, and result in hearing loss.

The most common types of bankruptcy filings are Chapter 7, Chapter 11, and Chapter 13

However, there are actually six types in practice today. These less-common options can be lifelines for foreign businesses operating in the US, local governments, and farms and fisheries.

A primer on reading the rock layers of roadcuts

Road construction often requires cutting through layers of rocks, coincidentally exposing millions of years of rock formation. This simple guide introduces basic rock types, what causes curves and folds, and the major rock formations in regions around the US.