ON December 5, 1872, about 400 miles west of Lisbon, the American ship Dei Gratia plunged headlong into the strangest conundrum in the history of shipping. Around three in the afternoon, it came abreast of a 103-foot American brigantine called Mary Celeste, which appeared to be unmanned and drifting. Captain Morehouse sent three sailors to board the ship. What they found on board has never been explained satisfactorily.