The construction of artificial islands off the coasts of the United States must take place within a framework of state, federal, and international law. Some guidance to the applicable law can be obtained from existing U.S. and state statutes that cover various activities in territorial waters and in the high seas beyond, as well as on the continental shelf. But many legal questions about the authorization and operation of artificial islands and the day to day issues of taxation, contracts, or tests on artificial islands can only be answered by speculations based on reasonable legal analogies and political likelihoods.
Artificial islands could be located within the three-mile territorial sea of the United States; or they could be placed beyond the territorial sea upon the continental shelf of the United States at distances anywhere from three miles to hundreds of miles out from shore. The first legal consideration from an international point of view is whether the artificial island lies within U.S. territorial waters or not; the second legal consideration is whether it lies upon the U.S. continental shelf, and whether the island can be justified by its connection to exploring or exploiting the resources of the continental shelf. In general, under international law, states may place islands for any purposes within their territorial waters subject to certain navigation easements. States may also place islands upon their continental shelves for purposes of exploring or exploiting the resources of the shelf. There is no rule of international law expressly prohibiting a state from the construction of islands of any type upon its continental shelf, but there are well-established rules enjoining any attempt to subject any part of the high seas to its sovereignty and ensuring the freedom of navigation, fishing, laying of pipelines and cables, and overflight to all states on the high seas. The major legal consideration of the location of an artificial island from an American point of view is whether it lies within the territorial waters of the United States, in which case it comes under the constitutional jurisdiction of the states as well as the federal government, or whether it lies beyond the territorial waters, in which case it comes under the jurisdiction of the federal government except as statute may provide for the jurisdiction of the states. Another general legal consideration is the definition of an artificial island. If the structure should be defined as a "vessel," then the location of the structure would be largely irrelevant insofar as vessels are governed by general maritime law as developed by federal statutes and interpreted or applied by federal (admiralty) courts. It must be recognized that the present legal situation both internationally and nationally is in flux. The United States, for example, has already signified its willingness at the Third U.N. Conference on the Law of the Sea in 1973–74 to accept a 12-mile territorial sea, provided international agreement can be reached on the free transit of international straits. Proposals to allow all coastal states of the world, moreover, to exercise jurisdiction over a 200-mile economic zone for all economic and conservation purposes have received wide support.