An artificial reef is a human-created underwater structure, typically built to promote marine life in areas with a generally featureless bottom, to control erosion, block ship passage, block the use of trawling nets, or improve surfing.
Many reefs are built using objects that were built for other purposes, such as by sinking oil rigs (through the Rigs-to-Reefs program), scuttling ships, or by deploying rubble or construction debris. Other artificial reefs are purpose-built (e.g. the reef balls) from PVC or concrete. Shipwrecks may become artificial reefs when preserved on the seafloor. Regardless of construction method, artificial reefs generally provide hard surfaces where algae and invertebrates such as barnacles, corals, and oysters attach; the accumulation of attached marine life in turn provides intricate structures and food for assemblages of fish.
The construction of artificial reefs began in ancient times. Persians blocked the mouth of the Tigris River to thwart Arabian pirates by building an artificial reef and during the First Punic War the Romans built a reef across the mouth of the Carthaginian harbor in Sicily to trap enemy ships within and assist in driving the Carthaginians from the island.
Artificial reefs to increase fish yields or for algaculture began no later than 17th-century Japan, when rubble and rocks were used to grow kelp. The earliest recorded artificial reef in the United States is from the 1830s, when logs from huts were used off the coast of South Carolina to improve fishing. In the Philippines a traditional native fishing technique known as fish nests (natively known by various names like gango, amatong, or balirong), is basically an artificial reef. It uses rocks and waterlogged wood to build mounds inside excavated trenches on shallow tidal waters that attract fish and crustaceans. The mounds are then harvested every few weeks during low tide by surrounding them with nets and dismantling them piece by piece. They are rebuilt after every harvest. Fish nests are often used to capture grouper fingerlings to be used as seeds for aquaculture. Fish nests were in common use since before 1939.
Beginning before the 1840s, US fishermen used interlaced logs to build artificial reefs. More recently, refuse such as old refrigerators, shopping carts, ditched cars and out-of-service vending machines replaced the logs in ad hoc reefs. Officially sanctioned projects have incorporated decommissioned ships, subway cars, battle tanks, armored personnel carriers, oil drilling rigs and beehive-like reef balls.
Artificial reef communities tend to develop in more or less predictable stages. First, where an ocean current encounters a vertical structure, it can create a plankton-rich upwelling that provides a reliable feeding spot for small fish such as sardines and minnows, which draw in pelagic predators such as tuna and sharks. Next come creatures seeking protection from the ocean's lethal openness—hole and crevice dwellers such as grouper, snapper, squirrelfish, eels and triggerfish. Opportunistic predators such as jack and barracuda also appear. Over months and years the reef structure becomes encrusted with algae, tunicates, hard and soft corals and sponges.
3D printing technology has been employed both to create moulds for cast ceramic and concrete artificial reefs, and to directly create artificial reefs, also through the use of environmentally friendly materials.
An electrified reef is an artificial reef where an small low voltage electric charge is applied to a sub-sea metallic structures that causes limestone to precipitate onto a metal frame onto which coral planulae can then attach and grow; the process also speeds up post-attachment growth.
Artificial surfing reefs have been created in several locations. Supporters cite benefits such as coastal protection, habitat enhancement and coastal research. The world's first attempt was made in El Segundo, near Los Angeles, in California. The next attempt was at Mosman Beach, Perth, Western Australia. This reef was constructed of large granite rocks placed in a pyramidal shape to form an appropriate breaking wave form that would suit surfers. An artificial reef constructed of over 400 massive, geotextile bags (each larger than a bus) filled with sand was constructed in 2000 at Narrowneck on the Gold Coast of Queensland, Australia. This artificial reef had two objectives: stabilizing beach nourishment and improving surfing conditions.
Europe's first artificial reef was approved in 2008. Construction began August 30, 2008, in Boscombe, Bournemouth, UK (5 months after 3 local councillors spent 18 days in New Zealand on a fact-finding mission), and opened in November 2009. The £3 million (2.5% of the Council's annual budget that year) reef was expected to create waves up to 30% larger and double the number of surfing days annually. Construction on this reef continued from June 2008 through August 2009. Boscombe Reef was built from large sand-filled geotextile containers, totaling 13,000 cubic metres (460,000 cu ft). It failed entirely and attempts were made to convert it into a multi purpose reef, which also failed. Bournemouth Council attempted to get a refund from the New Zealand-based reef construction company but it went into administration before paying any compensation.
In the United States coastal permitting requirements present major obstacles to building surfing reefs. The only reef built in the U.S. for surfing is southern California's "Pratte's Reef", which was constructed in 2000 and removed in 2008 as planned.
According to environmental group The Ocean Conservancy, the Osborne Reef may be an indication that the benefits of artificial reefs need to be re-examined. Jack Sobel, a senior scientist at the group, stated "There's little evidence that artificial reefs have a net benefit," citing concerns such as toxicity from paint, plastics parts, etc., damage to ecosystems and concentrating fish into one place (worsening overfishing).
Artificial reefs can show quick increases in local fish population rehabilitation, coral reef and algae growth. However, far more than half the amount of biomass found on artificial reefs is attracted from other areas rather than developing there. James Bohnsack, a biologist with the National Marine Fisheries Service (NMFS) concluded artificial reefs do not increase fish populations. Instead they operate as fish aggregating devices (FADs) bringing in fish from other reefs. Concentrating fish on a reef also makes for easier fishing.
The fish attracted to artificial reef zones vary from reef to reef depending on its age, size and structure. Large reef structures such as large sunken ships attract larger fish.
The use of shipwrecks in rocky zones creates a new trophic structure for the local ecosystem. They become the home for certain species and many nearby animals migrate to the shipwreck. This unbalances the natural ecosystem and has the potential to alter many other habitats.
Thousands of popular wreck diving sites throughout the world are shipwrecks sunk as artificial reefs. Some of these wrecks were sunk deliberately to attract divers. The USS Spiegel Grove and USS Oriskany in Florida, USS Indra and USS Aeolus in North Carolina, and Bianca C. in Grenada draw thousands of divers annually.
The materials used in most artificial reefs often cause pollution by releasing chemicals and nutrients that are not naturally found in reef environments. Ships can release polychlorinated biphenyls (PCBs), asbestos, iron, lead paint and anti-fouling paint leaches into the ocean and enters the food chain.
Waste tires were used to create many artificial reefs. In Florida alone, 571 permitted artificial reefs exist, and the number of illegal reefs is thought to be much greater. Tires are made from many chemicals and compounds such as black carbon, sulphur, zinc oxide and peroxides. The US National Artificial Reef Plan states that tires are a good reef construction material because no toxic substances are released from the decomposition of tires; though there is little information published to back up the claims and the future decomposing of the many different types of rubber tires could create unseen pollution.
Tires have fallen out of favor with marine biologists. Tropical storms may demolish the tire containment system, washing tires onto beaches, destroying nearby coral reefs and inhibiting new coral growth. As a result, states such as Florida and the country of France have begun large-scale removal of tire reefs. On the Osborne Reef off the coast of Fort Lauderdale, Florida boaters were allowed to dump unsecured tires at the site. Storms broke the nylon straps holding the original tire bundles together. As of November 2019, 250,000 of an estimated 700,000 tires have been removed. At other artificial reef sites hurricanes pushed tires up on beaches from Florida to North Carolina, damaging reefs, causing pollution and requiring costly cleanup. As a result, the Ocean Conservancy now includes tire removal during the International Coastal Cleanup in September of each year.
Some artificial reefs are used to prevent coastal erosion. They can be designed to act in multiple ways. Some are designed to force waves to deposit their energy offshore rather than directly on the coastline. Other reefs are designed to hold sediment on beaches. These reefs trap the sediment. These reefs are custom-designed for each unique zone.
In the early 1970s, more than 2,000,000 used vehicle tires were dumped off the coast of Fort Lauderdale, Florida to form an artificial reef. However, the tires were not properly secured to the reef structures, and ocean currents broke them loose, sending them crashing into the developing reef and its natural neighbors. As of 2009, fewer than 100,000 of the tires had been removed after more than 10 years of efforts.
Neptune Memorial Reef was originally conceived as an art project called The Atlantis Reef Project and was envisioned and created by Gary Levine and Kim Brandell. Burial at sea became a way of financing the project. As of 2011, about 200 "placements" had occurred. Cremated remains are mixed with concrete and either encased in columns or molded into sea-star, brain-coral, 15 feet (4.6 m) castings of lions or other shapes before entering the water.
In 1921 the US battleship Massachusetts was scuttled in shallow water off the coast of Pensacola, Florida and then used as a target for experimental artillery. In 1956 the ship was declared the property of the state of Florida by the Florida Supreme Court. Since 1993 the wreck has been a Florida Underwater Archaeological Preserve and is included in the National Register of Historic Places. She serves as an artificial reef and recreational dive site.
The second-largest artificial reef is USNS Hoyt S. Vandenberg, a former World War II era troop transport that served as a spacecraft-tracking ship after the war. The Vandenberg was scuttled seven miles off Key West on May 27, 2009, in 140 feet (43 m) of clear water. Supporters expected the ship to draw recreational divers away from natural reefs, allowing those reefs to recover from damage from overuse.
The ex-USS Spiegel Grove is located on Dixie Shoal, 6 miles (9.7 km) off the Florida Keys in the Florida Keys National Marine Sanctuary. Her exact location is .
USS Aeolus was sunk to form an artificial reef in August 1988. The ex-Aeolus, located about 22 miles from Beaufort Inlet in 110 feet (30 m) of water, is regularly visited by divers.
In late 2000s, the New York City Transit Authority decided to retire an outdated fleet of subway cars to make room for new R142 and R142A trains. The obsolete subway cars, (nicknamed "Redbirds"), had run on the A Division (former Interborough Rapid Transit Company routes) of the New York City Subway system for 40 years. Each car was stripped, decontaminated, loaded on a barge, and sunk in the Atlantic Ocean off the coast of Delaware, Virginia, South Carolina, Georgia, and Florida. Some cars had number plates removed because of rust, which were then auctioned off on eBay. A total of 1,200 subway cars were sunk for this project.
In September 2007, the NYCTA approved a further contract with Weeks Marine worth $6 million, to send 1,600 of its retired subway cars to be used as artificial reefs. The old models were sheathed in stainless steel, except for the fiberglass reinforced plastic front ends, which were removed before sinking. The retired fleet included old work trains and cars that were badly damaged beyond repair.
Since November 2009, artist Jason deCaires Taylor has created more than 400 life size sculptures off the coast of Cancun, Mexico at the Cancun Underwater Museum. The coral reefs in this region suffered heavy degradation due to repetitive hurricane abuse. This project was funded by The National Marine Park and the Cancun Nautical Association. It was designed to emulate coral reefs using a neutral ph clay. Taylor constructed unique settings depicting daily activities ranging from a man watching TV to a 1970s replica of a Volkswagen Beetle. This artificial reef relieved pressure from the nearby Manchones Reef.
Since the late 1990s, the Australian government has been providing decommissioned warships for use as artificial reefs for recreational scuba diving. So far, seven ships have been sunk:
The former ocean liner SS American Star was wrecked on 18 January 1994 on the west coast of the island of Fuerteventura, on Playa de Garcey. By 2008, the wreck of the ship had completely collapsed and sunk, and is now lying about 10 metres underwater near the coast. It's coordinates are 28° 20′ 45.88″ N, 14° 10′ 49.59″ W.
The Gibraltar Reef was first proposed by Eric Shaw in 1973. Initial experiments with tires proved unsuccessful as the tires were swept away by currents or buried underneath sand. In 1974, boats from local marinas and the Gibraltar Port Authority were donated. The first two were barges that were sunk in Camp Bay. In 2006, a 65-ton wooden boat, True Joy (also referred to as Noah's Ark) was sunk there as well, followed by MV New Flame, a mid-sized bulk carrier, in 2007.
In 2013, more than 70 concrete blocks were sunk, each one square meter in size with protruding metal bars. This led to heated debate between the United Kingdom and Spain, with Gibraltar accusing Spain of over forty incursions into their waters per month and Spain accusing Gibraltar of including metal bars in the reef to stop Spanish fishermen trawling the seabed for fish. The dropping led to a diplomatic conflict between the two countries because Gibraltar is a British Overseas Territory.
The surfing reef Temple reef is off the coast of Pondicherry, India constructed of fully recycled materials such as concrete, rocks, trees, palms, and iron bars. It is located at a depth of 18 metres (59 ft).
Pearl of Dubai is an art-inspired Lost City off the coast of Dubai. The site encompasses five acres and is located at the World Islands. At a depth of 10 to 20 metres (33 to 66 ft), the site is designed as an ancient lost city, complete with temples and statues using regional design cues from 800 BC.
Jordan made an under-water military vehicles museum, which is intended to form an artificial reef over time.
Underwater Chocolate Hills is an artificial reef project undertaken by Spindrift Reefs Dive Center off the coast of Panglao Island in the Philippines. It consists of broken coral harvested by local divers, who attach it to wire structures. The structures are built in the same shape as the Chocolate Hills, which can be found in the Bohol Region. The intent is to create a new dive site and new marine habitat.
Cooper Reef is a purpose-built artificial reef off the coast of Esperance, Western Australia. It is at a depth of 30m and consists of 128 dome-like concrete modules designed to attract fish and enhance fish stocks, thereby creating new fishing and recreation opportunities for tourists, anglers and local families.
A study in Barbados showed a marked variation in diver satisfaction with artificial reef diving experiences. Novice divers tended to be more satisfied than more experienced divers, who had a strong preference for natural reefs and large shipwrecks. 
Reef project gone wrong; crews now working to remove mass collection of tires