Never swim if you see this beach wave
Beachgoers who see "square waves" while on holiday should get out of the water immediately, experts warn.
Also known as cross seas, they may look pretty but are actually extremely dangerous - and have even caused shipwrecks.
Square waves can be found out in the open ocean as well as near the coast, and are formed by waves moving in opposite directions.
This happens when two separate weather systems collide.
The waves then create a chequerboard effect on the sea, with a grid system of squares on the surface.
They look gentle from above, but are actually similar to rips, which are extremely hard to escape from.
Anyone caught in the water is likely to struggle to escape them, as they will be fighting two different waves at the same time.
According to the European Space Agency, cross waves in open waters have even caused a number of shipwrecks over the years due to the battling waves.
For tourists wanting to see the phenomenon, they can often be found in France on the Ile de Re, according to Travel + Leisure.
Located near Rochelle, tourists can climb the lighthouse to see the waves from above.
This article originally appeared on The Sun and has been republished with permission
Originally published as Never swim if you see this beach wave