NEXT time you head to the beach, if you see "square waves" you should get out of the sea as soon as possible.

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 affect on the sea, with a grid system of squares on the surface.

They look gentle from above, but are actually similar to rip tides, 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, they have even caused a number of shipwrecks over the years due to the battling waves.

If you do want to see the waves – from a safe distance on land – they can often be spotted in France on the Isle de Re, according to Travel + Leisure.

Located near Rochelle, tourists can climb the lighthouse to see the phenomenon from above.

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