Scientists and concerned beachcombers are looking for answers.

Over the past several months, beachcombers on both sides of the Atlantic have been encountering the same thing – hundreds of Nike running shoes washing up on the sand, in rough shape after a lengthy sea journey but appearing to be unworn, all with the same manufacturing date. Other footwear, including flip-flops, have been found with increasingly frequency as well.

Despite being found on beaches in the Bahamas, Bermuda, the Azores, Brittany, Cornwall, the Orkney Islands, and on the west coast of Ireland, the consensus is that all the sneakers and flip-flops come from the same source. A ship called the Maersk Shanghai was traveling between Norfolk, Virginia, and Charleston, South Carolina, in March 2018 when it met a storm and lost 16 shipping containers overboard. Nine were rescued, but seven sank, presumably expelling their contents.

Neither the shipping company nor Nike have confirmed that this is the actual source of the shoes, but two other footwear manufacturers, Triangle and Great Wolf Lodge, whose products have been found on beaches, did say that they lost merchandise from the Maersk Shanghai.

It’s an interesting story because it reveals a few things – first, how hidden from public view the world of shipping mishaps is. Companies do not have to report what happens. According to the BBC.

“Shipping companies only have to report lost containers if they could become a hazard for other vessels or if they include substances deemed ‘harmful to the marine environment’, such as corrosive or toxic chemicals. While the Marine Conservation Society says products like trainers harm marine environments, they do not count as ‘harmful’ for the purpose of reporting cargo lost at sea.”

Second, it provides insightful information about ocean currents. While a few have washed up on beaches, most of the shoes are probably doing laps around the Atlantic. The BBC cites Dr. Curtis Ebbesmeyer,

“If they’ve gone about halfway around [from North Carolina to the UK] in just over a year, then it takes about three years to go once around the North Atlantic. So that’s the typical orbital period of the sneakers, but that hasn’t been studied by oceanographers much at all.”

Ebbesmeyer also observed that left and right sneakers tend to go in different directions, floating “with different orientation to the wind.”

While news of overboard containers may travel quickly, companies expect it to die out just as fast and for the public to forget. But when the evidence keeps washing up on shore, this is impossible. The beach-cleaning community is calling for stricter laws that would require shipping companies to admit exactly what’s getting lost at sea. This would likely spur them to improve cleanup strategies, as well, instead of hoping the problem will simply disappear beneath the waves.