Diver looking at artificial reef in Permuteran Bay, Bali Island, Indonesia. The man-made metal Bio Rock reef structure has a low-voltage electrical current flowing through the metal frame. This causes minerals in the sea water to precipitate onto the steel frames, resulting in the formation of a limestone layer. Scientists then attach small pieces of coral to the Bio Reef structures. These corals quickly become cemented into place by the accumulating limestone. Indonesia, tropical Pacific Ocean.