the shells of 200 New York subway cars were sunk on the Florida coast in an experiment to provide new habitat for marine life. It worked pretty well evidently

https://www.postandcourier.com/environment/20-years-ago-sc-reef-system-added-nyc-subway-cars-today-sea-life-passengers-thrive/article_db26a94c-2a48-11ee-8d65-c7dde08cb827.html