Most of the world’s everyday goods and raw materials moved over long distances are packed in large metal boxes the size of tractor-trailers and stacked on ships. Millions of containers cross the oceans every year. But not everything arrives as planned. According to one trade group, at least 20,000 shipping containers have tumbled overboard in the last decade and a half. But tracking efforts are incomplete. A few shipwrecks and disasters grab headlines, like the March crash of a cargo ship into a Baltimore bridge. But much less is known about how often containers are lost piecemeal or away from major ports.