From National Geographic:
People around the world have long known that shipwrecks are prime fishing sites, and since at least the 1830s, American fishermen purposely built artificial reefs out of interlaced logs. In our own time the materials of do-it-yourself reefs have tended to be castaway junk: old refrigerators, shopping carts, ditched cars, out-of-service vending machines. Pretty much anything you can sink has the potential to become an artificial reef. Even officially sanctioned ones are often created from distinctly odd materials, including decommissioned subway cars, vintage battle tanks, armored personnel carriers, oil drilling rigs, and specially designed beehivelike modules called Reef Balls.
All of the photos are in the February 2011 issue of National Geographic magazine, on newsstands now. They can also be found on National Geographic's website.