If you have ever been in the ocean or walked along the beach, and saw waves forming a grid pattern moving across the ocean, then you have witnessed a dangerous phenomenon known as square waves or cross seas.
A fascinating but dangerous phenomenon
Most of us are familiar with waves that reach the shore and break horizontally while being parallel to the coastline, but in some parts of the world you will find that waves seem more like a chessboard on the water surface. This is an extremely rare type of wave, but it remains one of the most dangerous. If you look at the waves from the sky, you might think that they form a grid underwater.
And this thought would not be wrong, there is a square wave current that forms under water. This strange pattern is quite easy to spot, so if you see it, you should immediately become aware of the danger. This is because they are not the result of currents forming in the depths, but of the way the water moves.
A perfect geometry to be observed from afar
To break it down as simply as possible, square waves are what happens when two seas meet, it is the intersection of these two seas that collide. This means that two opposing swells collide, forming the unique square pattern. And although this may sound impressive, ship captains know that these areas must be avoided at all costs. The phenomenon is usually accompanied by powerful retaliation, even in small swells.
They are known to cause boating accidents and shipwrecks, and even experienced swimmers or wave enthusiasts meet there regularly. The problem is that the cross waves are much too strong to get out and tend to occur extremely quickly, appearing and disappearing before you can react. A famous place to observe square waves is on the Ile de Ré, where the phenomenon is a tourist attraction despite the dangers and their rare appearances.