Have you ever seen the ocean form what looks like a giant checkerboard? These unusual patterns are known as square waves, or cross seas, and they occur when two different wave systems travel across each other, sometimes at nearly right angles. Although they look fascinating from the shore, they can indicate turbulent and complicated sea conditions.

Square waves form when swells generated by different weather systems meet in the same area. Instead of moving in one direction, the waves cross over one another, creating the distinctive grid-like pattern. The phenomenon can be temporary, but while it is happening, the water may be much more unpredictable than it appears.

That is why swimmers, surfers, and boaters should treat these conditions seriously. Cross seas can involve strong currents and rapidly changing waves, making it difficult for swimmers to maintain control or return to shore. For smaller vessels and paddleboards, waves approaching from different directions can also make stability more difficult. If you see a pronounced cross sea, the safest choice is to stay out of the water.

If you’re already swimming or wading when the pattern develops, calmly leave the water and move to a safe place on shore. Never assume that calm-looking conditions between waves mean the water is safe. Square waves are one of nature’s most fascinating visual phenomena—but they are a reminder that the ocean can change quickly, and beautiful water isn’t always safe water.