Have you ever spotted a quadrilateral-shaped wave pattern in the ocean? If so, you've witnessed square waves.

Most of the waves reach the shore and break horizontally and parallel to the coastline.

However, in some regions of the world, you may notice the creation of a chessboard-like pattern on the surface of the water.

The phenomenon is rare, yet extremely dangerous. If you look at these waves from above, it seems like there's a grid underwater forming square-shaped ripples.

You easily can spot the strange pattern by climbing to the top of a lighthouse, cliff, or by flying a drone over the area.

However, square waves are not a result of any circumstance that is taking place below the water. Instead, they're the result of the way the water is moving.

In other words, square waves are a consequence of the intersection of two seas, also known as cross sea or grid waves.

A weather pattern in the region causes the waves to form this way and at different angles. When two opposing swells collide, a unique pattern emerges.

Cross Seas Are Extremely Dangerous

The overall look of square waves is impressive but, in fact, they're extremely dangerous, even in small swell conditions.

If you see square waves in the ocean, get out of the water as soon as possible. Why? Because the phenomenon is usually associated with strong and powerful rip tides.

Swimming or surfing in the middle of a cross sea is not something you should be doing, even if you're an experienced swimmer or wave rider.

Square waves can also cause boating accidents and shipwrecks.

The square-shaped waves are fascinating, but these criss-cross ripples pose a real threat to everyone and are too strong to get out of.

Cross seas are relatively rare and can actually appear and disappear within minutes on many beaches around the world.

The most famous place in the world for watching square waves is Île de Ré - Isle of Ré - in France.

The island, located off the coast of La Rochelle, is a tourist attraction even though cross seas only occur during certain times of the year.