Artificial reefs can be built in a number of different methods. Many reefs are built by deploying existing materials in order to create a reef. This can be done by sinking oil rigs (through the Rigs-to-Reefs program), scuttling ships (such as the USS Oriskany), or by deploying rubble, tires, or construction debris. Other artificial reefs are purpose built (e.g. ASR or reef balls) from PVC and/or concrete. Historic or modern shipwrecks become unintended artificial reefs when preserved on the sea floor. Regardless of construction method, artificial reefs are generally designed to provide hard surfaces to which algae and invertebrates such as barnacles, corals, and oysters attach; the accumulation of attached marine life in turn provides intricate structure and food for assemblages of fish.