NEW YORK - Some 250 million containers cross the oceans every year, but not everything arrives as planned -- more than 20,000 shipping containers have tumbled overboard in the last decade and a half, according to the World Shipping Council (WSC), an industry group.

"Their varied contents have washed onto shorelines, poisoned fisheries and animal habitats, and added to swirling ocean trash vortexes. Most containers eventually sink to the sea floor and are never retrieved," said The Associated Press (AP) in its report on Thursday about the data.

ALSO READ: Shippers scramble ahead of looming US East Coast port strike

Cargo ships can lose anywhere from a single container to hundreds at a time in rough seas. Experts disagree on how many are lost each year. WSC reported that, on average, about 1,500 were lost annually over the 16 years they've tracked, though fewer in recent years.

Others say the real number is much higher, as the shipping council data doesn't include the entire industry and there are no penalties for failing to report losses publicly, according to the report.