It's bigger than Central Park, is 24 metres down in the ocean – and contains hundreds of subway cars – and a few tanks

Bearing in mind that there could be around three million sunken vessels lurking at the bottom of the ocean, it makes sense that shipwrecks are the most common type of artificial reef.

On a site the size of 27 football fields, 2 million of these were dropped in the ocean to create an artificial reef – instead, it became a disaster
"At first, they are little more than bare surfaces in the ocean’s vast expanse.” Why do sunken ships make good artificial reefs?

But boats aren’t the only things that have been used to create new habitats. There are even decommissioned subway carriages beneath the waves.

Possibly the most famous of these submerged subway reefs is Redbird Reef. It’s around 24 metres deep off the coast of Slaughter Beach, Delaware.

It's longer than the Leaning Tower of Pisa, weighs 499 tonnes and sits 24 metres below the surface, where sharks patrol Noonu Atoll’s tropical waters

The creation of Redbird Reef began in 1996 as part of a wider programme using recycled materials to create new habitats across several artificial reef sites.

“These materials include more than two million tons of rock, 100,000 tons of concrete, 86 tanks and armoured personnel carriers, upwards of 1,350 retired subway cars and 32 retired vessels ranging from 40 to 653 feet [12 to 199 metres] long,” says the Delaware Department of Natural Resources and Environmental Control (DNREC) on its website.

The New York subway cars are the reef’s star attraction.

Hundreds of retired Redbird subway carriages – which lend their name to the reef – were scuttled, after any hazardous materials, doors and windowpanes were removed. More were sunk in other areas of the Delaware artificial reef programme and beyond in other states.

But that’s not all that’s down there. Tugboats, trawlers, barges, warships and a cruise ship are all spending their retirement as part of the area’s artificial reefs.

Before the creation of the Delaware Artificial Reef Programme, the area was largely made up of sandy seabed. Now, the reef structures attract lots of fishes including sea bass, tautog, triggerfish, flounder, tuna and even sharks.

According to the 2023–4 Delaware Reef Guide, the artificial reef programme brings around six million US dollars into the local economy each year thanks to the fishing and diving opportunities it creates.

Despite Redbird Reef’s fame, not everyone is fully convinced by its positive environmental impact. A 2016 AGU paper studied how storms have impacted the structures and found that 2012’s superstorm Sandy damaged or destroyed eight subway cars while storm Jonas “rotated a previously static 47m long Navy barge nearly 60 degrees” in 2016.

The fragments from damaged structures can become problematic. “Once destroyed or collapsed by waves the subway cars go from providing positive structural relief and thus beneficial habitat above the surrounding seabed to being reduced to scattered low relief marine debris,” write the study authors.

It’s a “cautionary tale”, they say, reminding people to carefully consider the materials they use to create artificial reefs and how they might be impacted by currents, waves and storms.

Famously, this is something that wasn’t fully considered when the Osborne Reef was created in Florida in the 1970s. The project was supposed to help recycle disused tyres but ended up as an environmental catastrophe. Decades later, cleanup projects are still underway. A stark reminder of the importance of proper research and planning before putting things into the ocean, even if the act is well-intentioned!