Since 2001, 714 retired New York City subway cars have been sunk off Delaware's coast to seed an artificial reef; within seven years the marine food per square foot there had increased 400 times - parade.com

An ambitious underwater experiment off Delaware’s coast has yielded striking results, with retired New York City subway cars repurposed as artificial reefs dramatically boosting marine life. Since the project began in 2001, the submerged cars have created thriving ecosystems, leading to a 400-fold increase in food per square foot within just seven years. Marine biologists cite the structure’s ability to attract diverse species, transforming a barren seabed into a productive habitat. The success raises questions about innovative solutions for both urban waste disposal and marine conservation.

Since 2001, 714 retired New York City subway cars have been sunk off Delaware's coast to seed an artificial reef; within seven years the marine food per square foot there had increased 400 times  parade.com