What is probably the world's richest sunken treasure — the Sussex, a British warship that went to the bottom of the Mediterranean in 1694 with a cargo of coins now worth up to $4 billion — has become embroiled in a bitter diplomatic dispute that pits Spain against Britain, the United States and an American company that wants to salvage the wreck.

The conflict turns on arcane and often disputed aspects of international law that govern sovereign waters and the rights of shipwreck owners and finders.

Spain claims the waters, off the coast of Gibraltar. Britain claims the ship, says its decomposing hull rests in the high seas, and has struck a deal with the American company, Odyssey Marine Exploration Inc. of Tampa, Fla., to split the recovery's proceeds.