Answer: Subway Cars

Off the coast of Delaware, roughly sixteen miles off the mouth of the Indian River inlet in the Atlantic Ocean, is the most popular artificial reef in the region: Redbird Reef. The reef owes its name to the Redbird subway car, an ubiquitous feature of the New York City subway system throughout the later 20th century.

After the cars were completely phased out in the early 2000s, they were donated to serve as the seeds for artificial reefs up and down the  Eastern U.S. seaboard. The first of these installations was in Delaware and consisted of over 700 subway cars. Subsequent locations include artificial reefs off the coast of Georgia, South Carolina, Virginia, and New Jersey.