His expedition found a “mountain of gold” at the bottom of the Atlantic. But he might never see a cent of the lost fortune. It was late in the summer of 1988 when Tommy Thompson finally located a big rusty paddle wheel buried in ocean floor sediment. Now he was banking on an even bigger discovery: a cache of 19th-century gold worth as much as half a billion dollars. On September 11, Thompson and a crew of explorers hunkered down aboard the Arctic Discoverer, a retrofitted Canadian icebreaking ship about 160 miles east of South Carolina. There they deployed an...