With summer on its way and many of us planning to head abroad or to the coast, there's a dangerous nautical phenomenon that swimmers need to be aware of.
Once thought by some to be a sign that an earthquake or tsunami was imminent, square waves – or cross seas – are caused by two separate weather systems colliding and creating an almost chessboard-like pattern upon the water.
But, no matter how weirdly pretty or inviting they may appear, they can actually prove fatal to any unsuspecting beachgoer who might fancy a dip.
That's because, underneath the surface, they're indicative of the presence of strong riptides which can pull in two different directions, thereby proving very difficult to break free from should a swimmer get caught up in them.
It's also a phenomenon which is far more common than you might think and has even been attributed to a number of boating accidents and shipwrecks over the years.
This is likely due to vessels taking on water from waves coming at them from two different directions at once and the fact that the resulting swells can reach up to 10 feet high.
The place where they seem to be most prevalent is off the coast of La Rochelle in France, namely the Isle of Ré – the spectacle often attracting a great number of tourists each year.
The topic also regularly causes a mix of wonder and panic online.
One Reddit user wrote, "So two wave fronts meet at the correct angle to produce an interference pattern that can hit ships from unconventional angles making them dangerous.
"This is is really cool, makes sense and is the first time I have seen this s**t."
Meanwhile, another posting on Quora.com added, "If you are in a very small craft then you can't run and you can't hide. The harmonic currents are really going to make it hard to navigate.
"Rip currents will drag you under if you try to swim for it. You may be really in big trouble if you are on a surfboard as well. Try praying."
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