Huge concrete sea structures are being conceived for oil storage, for ponding of fish and for offshore floating nuclear power plants. Some are already being designed and built. The first to come to reality is a part of Ekofisk City: a two acre concrete island supported by a million gallon prestressed concrete oil storage tank resting on the bottom of the sea. Ekofisk City is the Phillips Petroleum Group's oil producing and shipping complex located over a Norwegian oil field at the center of the North Sea. Built almost entirely in harbor, the tank was towed about 200 miles over a ten day period and, on June 30, settled into its final position in water 230 feet down. The two platforms above the tank will provide space for three permanent cranes, a helicopter landing pad, oil processing equipment and a gas-fired power plant that will furnish enough energy to supply power for all of Ekofisk City with some left over for France and England.