Practically all of the mineral and energy resources found on land are present under the sea. Development, however, is limited by extraction costs that increase with depth of water, by the relative abundance of resources on land, and by political questions involving ownership of deep ocean resources.

Worth $80 billion, the most valuable undersea commodity is oil and natural gas, representing 90% of the resources obtained from the seafloor. Continental-margin deposits comprise about one-third of the world's estimated oil and gas reserves. Large deposits have been found on the continental shelves in the Gulf of Mexico, the Persian Gulf, the North Sea, and off the coasts of northern Australia, southern California, and the Arctic Ocean. Other sites are promising, and many other continental shelves remain relatively unexplored. The harsh conditions found in the North Sea have fostered huge investments in colossal offshore rigs. Extraction costs on the continental...