If you have ever been out in the ocean or walking along the beach, only to spot a quadrilateral shaped wave pattern moving across the ocean – then you have witnessed a dangerous phenomenon known as square waves.
Most of us will know of the waves that reach the shore and break horizontally while being parallel to the coastline, but in certain regions of the world, you will find that waves appear to be more of a chessboard shape on the surface of the water. This is an extremely rare type of wave to happen, but it remains one of the most dangerous. If you look at the waves from the sky you would think they are a grid forming underwater.
And this thinking would not be wrong, there is a square-shaped ripple current forming under the water. The strange pattern is pretty easy to spot, so if you do see it, then you should immediately be aware of the danger. This is because they are not the result of currents forming in the depths, they are the result of how the water is moving.
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 bumping into each other. This means that two opposing swells are colliding, which is what forms the unique square pattern. And though it may look impressive, ship captains know to avoid these areas at all costs.
The phenomenon usually comes hand in hand with powerful riptides, even during small swell conditions. They are known to cause boating accidents and shipwrecks, even experienced swimmers or wave riders are caught up in them regularly. The problem is that the criss-cross ripples are much too strong to get out of, and they tend to happen extremely quickly, appearing and disappearing before you can even react.
A famous place to watch the square waves is at the Isle of Rhe in France, where the phenomenon is a tourist attraction despite the dangers and that they only rarely appear.