Going on a trip to the beach is what holiday dreams are made of. A beautiful scene is lying in the sun with warm sand between your toes and the water lapping against your feet. If you want to enjoy the ocean, don’t forget to stay safe in the water. Watch out for waves, and never turn your back on the ocean.

You may know a bit about rip currents and changing tides, but you might not know how dangerous square waves are. Yes, this really does happen, and it’s a beautiful sight. But it’s also one of the scariest things you can see in the ocean.

A square wave, also called a “cross sea,” is made when two waves meet to make a square shape that looks a lot like a checkerboard. In 2010, the European Space Agency said, “The conditions are quite common in the ocean and occur when a windsea and a swell, or two swell systems, coexist.”

It pointed to a 2004 study that showed “a large percentage of ship accidents occurred in crossing sea states.” HowStuffWorks went on to state that these square waves don’t happen very often, but when they do, they tend to be near the coast. Along the western coast of France, on the Île de Ré, is a great place to see them from a safe distance.

It can be hard for boats and swimmers to get through these cross-seas because they can make waves up to 10 feet high and change the way the wind blows. Again, this is very unlikely to happen, but if it does, don’t go out on a boat or swim in the water, because it might be rough.

You could instead spend your time sitting on the sand or taking a quick dip in the shallow water until the weather is better. Then you can go into the water safely and without crowds. What happens if you get caught in a square wave?

When you’re in the water, you might not notice that the waves are arranged in a grid, but you might notice that the swells get bigger and you have to swim against two different currents. Not swimming out so far in the first place is best, and getting out of the water as soon as the waves get too big is also best.