What are "square waves" and how dangerous they are for ships (3 photos)
Sometimes so-called "square waves" can form. Such a spectacle can amaze observers, because the water becomes looks like a chessboard. 
"Square waves" or, as they are more commonly called, crossed waves (from the English concept of cross-waves) is a rather rare phenomenon, in there are not many places in the world where this type of natural oddity can be observe.
The most popular places are the southwest coast of France, The Mediterranean Sea in the Tel Aviv area, less often - the Azores and Crimea.
The nature of this phenomenon is interesting. Even if you don't know correct answer, then it is enough to guess how this happens easy - and most often the guess will be correct. In this case, it is about the collision of two forces - the sea current and the wind. True, this and that should be enough, but at the same time and moderately strong. 
Most often, "squares" occur during a sharp change in the wind, when, you can say, the water does not have time to "rebuild". And sometimes this effect observed when two oncoming storms appear, which begin to drive water in two directions.
And you need to understand that such "squares" are most often not ripples on water, but rather high waves, sometimes reaching a height of several meters. 
This natural phenomenon is considered quite dangerous for ships, especially not very large ones. Since the waves do not hit from one side, but from two sides, the boat may sway violently and eventually capsize. AND it’s all the more impossible for people to swim in these cross waves - this deadly.
