In 2001, New York City had over 1,000 outdated subway cars on its hands. When they were first introduced in 1959, the old Redbird trains were gorgeous machines, but after four decades of service, it was time for the battered cars to be permanently retired. But rather than take them to a slag heap to be salvaged for scrap or crushed into little metal cubes, the city took 619 of the cars, stripped them of their windows and oily undercarriages, steam cleaned them, and then hauled the 20,000 pound metal boxes down to Delaware on a freighter ship. Then they dumped them all into the sea.
This post should also mention that this is a good thing, and not pollution. These cars will provide homes for sea life to live in, and act as a starting point for reefs to grow on.