COLUMBUS, Ohio (AP) — A former deep-sea treasure hunter is about to mark his fifth year in jail for refusing to disclose the whereabouts of 500 missing coins made from gold found in an historic shipwreck.

Research scientist Tommy Thompson isn't incarcerated for breaking the law. Instead, he's being held in contempt of court for an unusually long stretch — well past the normal maximum limit of an 18-month internment in cases of witnesses refusing to cooperate.

But nothing is usual about Thompson's case, which dates to his discovery of the S.S. Central America, known as the Ship of Gold, in 1988...