April 15 is observed by many as Titanic Remembrance Day. The ship struck an iceberg in the North Atlantic at 11:40 p.m. the evening of April 14 at a speed of 20 knots and slipped below the surface ...

19th-century engraving depicting the Santa María, the ship used by Christopher Columbus. Alamy For four centuries, Spain’s prodigious naval power built an empire that stretched around the globe.

“We like to picture shipwrecks as Greek temples on a hill—you know, very picturesque,” he told me. “But they’re not. They’re ruined industrial sites: piles of plates and rivets and ...