Shipwrecks have long fascinated oceanographer Robert Ballard, who became renowned for locating the wreck of the RMS Titanic in 1985. He talks with correspondent Lee Cowan about how that discovery 12,000 feet beneath the Atlantic changed him, and about the efforts of his research vessel Nautilus to map the ocean floor. Cowan also talks with Sophie Elmhirst, author of the bestseller "A Marriage at Sea: A True Story of Love, Obsession, and Shipwreck," about human beings at their most vulnerable.