Preference Center

We use cookies and similar technologies to collect device data and usage information for analytics, measuring the effectiveness of ads, personalizing content, and tailored advertising. By clicking “Accept”, you agree to such purposes and the sharing of your data with our partners. To learn more, CLICK HERE. To change your preferences at any time, click the “Do not Sell my Personal Information” link in the footer of this page.

Often mistaken for rocks or plants, corals are actually invertebrate animals. Found in tropical and subtropical waters, from the Caribbean to Hong Kong (pictured), these creatures are under threat from climate change. Scroll through to learn more about corals and their underwater world.
Phil Thompson
Almost all corals belong to an animal group called Anthozoa, which includes hard corals, soft corals, sea anemones and sea fans.
Andre Seale/VW PICS/Universal Images Group/Getty Images
Hard corals, or stony corals, have a rock-like appearance and are known as reef builders. Hard corals create a hard limestone exoskeleton which form reefs, like this honeycomb coral. They rely on tiny algae to survive, which also give the corals much of their color.
Reinhard Dirscherl/ullstein bild/Getty Images
Platygyra, known as "brain coral," is another of these hard corals. Its maze-like valleys attract marine life that can nest or hide from predators in its ridges.
Alexis Rosenfeld/Getty Images
Soft corals look more like plants than rocks; this gorgonian coral in the Philippines' UNESCO World Heritage site of Tubbataha has a tree-like appearance, and marine life often lives in its branches.
Alexis Rosenfeld/Getty Images
Both hard and soft corals are made up of hundreds or thousands of individual organisms called "polyps." Related to the jellyfish family, these polyps have tentacles -- multiples of six for hard corals, and eight for soft (pictured).
VisionDive/Adobe Stock
The Coral Triangle -- encompassing 2.3 million square miles which includes the oceans surrounding the Philippines, Malaysia, Timor-Leste, Papua New Guinea, the Solomon Islands and Indonesia (pictured) -- has the most coral diversity in the world, with over 600 species.
Parnupong Norasethkamol/Adobe Stock
Coral reefs cover less than 1% of the ocean floor, but more than 25% of the world's known marine life live there -- including more than 2,200 fish species in the Coral Triangle alone.
Alexis Rosenfeld/Getty Images
Most coral reefs are found between 30 degrees north and south of the equator in warm tropical waters. This includes the Caribbean (pictured) and the Great Barrier Reef in Australia, as well as South East Asia's Coral Triangle.
Reinhard Dirscherl/ullstein bild/Getty Images
While reef-building corals only live in shallow water, where the sunlight can reach the algae they coexist with, some corals like the spiral whip can thrive in deep water. Some corals have even been found 20,000 feet below the surface.
Avalon/Universal Images Group/Getty Images
But coral are under threat. Scientists predict that increasing ocean temperatures and water acidity may destroy up to 90% of coral reefs around the world over the next two decades. The increased temperatures lead to coral "bleaching", when the corals reject the algae they rely on to live, as seen at this reef in French Polynesia.
Alexis Rosenfeld/Getty Images
Coral reefs support underwater ecosystems, such as seagrass meadows, which are important carbon stores.
Reinhard Dirscherl/ullstein bild/Getty Images
Coral reefs provide economic benefits, too. Coral reefs support around 1 billion people through coastal industries including tourism and fishing, as well as providing coastal defense from storms.
Andre Seale/VW PICS/Universal Images Group/Getty Images
Inventing Tomorrow

How 3D printing could help save Hong Kong's coral

(CNN) — With a reputation as a concrete jungle, few know that Hong Kong is one of Asia's most biodiverse cities and boasts more hard coral species than the Caribbean.
But these corals are under threat from pollution and rapid urban development.
ArchiREEF, a spin-out company from Hong Kong University, says it has a solution: 3D-printed terracotta tiles designed to help corals grow and restore ocean life.
Co-founded in 2020 by marine biology professor David Baker and PhD student Vriko Yu, the company hopes it can help make corals "more resilient" against climate change.

Rock bottom

Found in warm, shallow water, coral reefs cover less than 1% of the ocean floor but host more than 25% of marine life. Corals, invertebrate animals that live in vast underwater colonies that form reefs, have existed in Hong Kong for thousands of years, but pollution and coral mining have decimated the population.
Baker and Yu have been trying to restore coral at Hoi Ha Wan Marine Park since 2016 in partnership with the Agriculture, Fisheries and Conservation Department (AFCD).
Hoi Ha Wan, now a protected coastal reserve, in the north of Hong Kong, used to be the site of coral mining for construction materials. This created a major barrier to restoration, says Baker as "the hard bottom of the seafloor has been mined away," leaving only sand and rubble.
The team needed to create a new "bottom" for the corals to grow on. Working with the university's architecture department, they began developing an artificial coral reef using the university's 3D printing facility.
This artificial reef is made from tiles that are roughly two feet wide, and mimic the natural shape of platygyra, known as "brain coral." Its twisting "valleys" attract marine life which can nest or hide from predators, says Yu.
A cuttlefish living in ArchiREEF's clay reef tile test site in Hoi Ha Wan Marine Park, Hong Kong.
Vriko Yu
Once placed in the water, the team attach baby corals to the tile with non-toxic glue. The shape of the tile helps the corals grow upwards, attracting marine life that build their homes in the reefs.
More than 130 of these tiles were installed on the seabed in Hoi Ha Wan Marine Park in the summer of 2020, and archiREEF was born.

Commercializing restoration

The 3D-printed tiles are just one small part of archiREEF's business model. After engaging corporate clients and governments to sponsor a restoration project, archiREEF will identify a site for restoration, install the reef tiles, and continue to manage the site for up to five years, monitoring the growth and the biodiversity of the reef.
Conventional artificial reefs, such as submerged concrete structures, often replace the function of the coral as a habitat for marine life, says Yu, whereas archiREEF wants to provide a "foundation" for the coral to grow on. Eventually, the coral will be strong enough to support itself without the tile -- which can then be shattered by divers, or will naturally erode.
Baker says that archiREEF can help regrow coral in areas where it has entirely disappeared, a situation which is becoming more common: scientists predict that warming ocean temperatures and increasingly acidic waters may destroy up to 90% of coral reefs globally in the next 20 years.
A diver positioning a 3D-printed terracotta tile on the sea floor.

An 'innovative' approach

Companies such as Australia-based Reef Design Lab and D-Shape have previously used 3D printing to create underwater habitats. But archiREEF's use of clay, a non-toxic and natural material, is "innovative," says Pui Yi Apple Chui, a coral restoration researcher at the Chinese University of Hong Kong.
Unlike concrete, clay is slightly acidic, like coral and has a similar chemical makeup to real reefs. The 3D printing technology also offers unlimited customization, which the architects at Hong Kong University say enabled them to create more places for the corals to grow and combat specific environmental problems with a bespoke design.
However, Chui also adds that people need to be "realistic" about restoration and consider the cost and accessibility of technology. Places like Hong Kong also need more data on the efficacy of conservation efforts before it can be scaled in a meaningful way, she says, adding: "Restoration should be the last resort; we should protect before we restore."

Conservation for everyone

Rescuing these reefs can help sustain the planet's underwater ecosystems, such as seagrass meadows, which benefit from coral reefs, and are important carbon stores.
There are economic benefits to coral reefs, too. According to a 2021 report from the Global Fund for Coral Reefs, coral reefs support the lives of around one billion people through coastal industries such as tourism and commercial fisheries, as well as providing coastal defense from storms.
Fossil evidence points to corals living in Hong Kong for thousands of years.
Phil Thompson
ArchiREEF still has a way to go before scaling up. The team will observe the test site in Hoi Ha Wan for at least another two years to ensure the corals continue to thrive and survive natural disasters, such as typhoons. However, Yu says the results at the test site so far are promising: in the first six months of observations, four times more coral has survived on the clay tiles compared to conventional artificial reefs.
A further 10 test sites are already planned, which the startup will use to refine the 3D-printing process and explore alternative materials, as producing clay is energy intensive and produces large amounts of carbon dioxide.
While archiREEF's focus is currently on government and large corporate clients, it hopes to one day engage individuals in its projects, too.
"Restoration is currently restricted to conservation scientists," Yu says. "But if we're talking about stakeholders, it's everybody."
Read more
More from CNN Travel