The phenomenon is thought to have caused many shipwrecks over the years.

By Marisa Spyker
June 27, 2019
Isle de Ré, France
Michel Griffon/Wikimedia Commons

Like sand on the beach and a breeze in the air, waves are an expected and welcome part of any trip to the sea. They’re fun to splash in, soothing to listen to, and thrilling to ride, if surfing’s your thing.

But anyone who’s ever seen The Perfect Storm or The Poseidon Adventure knows that not all waves are so friendly. And unfortunately, the ones that might cause danger don’t all come with a glaring warning sign on them.

That’s the case with the beautiful phenomenon of a cross sea. Formed both in the open ocean and nearer to coastlines, the rare event happens when wind from two separate weather systems collides, causing waves to move in competing directions. When the wave systems intersect, it creates a grid-like pattern in the sea, reminiscent of squares on a chessboard.

When on land, the unusual sight is cause for a photo opp. But while the waves might seem like they’re moving gently along, it’s definitely not something you’d want to be caught in. That’s because cross seas are notorious for creating powerful rip tides that are difficult to escape from. (Imagine having to dodge waves from both sides at one time while swimming.) According to the European Space Agency, cross seas are also thought to have caused a large number of shipwrecks over the years, with waves that can reach nearly 10 feet high.

While the unique wave patterns can occur anywhere, they tend to stick closer to shore and hang around for just a short moment. Isle de Ré, an island off the west coast of France, however, is said to have a better chance of seeing the rare squares due to its geology. So, if you’re hoping to witness this fascinating scene in person, you might want to head to Europe (and keep your eyes peeled at your hometown beach, too).

Just remember to stay out of the water.

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