The dangerous ‘cross sea’ warning sign that all beachgoers need to know

The next time you see latticed waves in the sea at the beach, get out of the water immediately.

Called the Cross Sea, they are so dangerous that they have even caused shipwrecks over the years.

A "cross sea" is very dangerous to swim in - and has even caused shipwrecks


A “cross sea” is very dangerous for swimming – and has even caused shipwrecksPhoto credit: Wikipedia/Michel Griffon
They are also known as square waves


They are also known as square wavesPhoto credit: YouTube/John Bern

Also known as square wavesthey can look interesting and are usually found in the opening bodies of water, although they can be spotted closer to shore.

Places where they have been sighted are Francealso on the Île en Re nearby la Rochelle as well as near Lisbon in Portugal.

Cross seas are caused by two separate weather systems colliding.

They are very similar in rip tides which are difficult to get out of due to the two different wave directions.

To recognize them, they appear at the top of the water by a checkerboard effect on the surface.

They have even caused a number of shipwrecks over the years, according to the European Space Agency, as the ships were set in motion by the different directions of the waves.

A German research icebreaker named FS Polarstern reported a dangerously serious rolling incident allegedly caused by a cross sea.

If you want to see the cross waves, it’s best to see them from land to see the full effect while staying safe.

People on social media shared their experiences with square waves – and how dangerous they are.

One person on YouTube said, “Growing up in Hawaii, we were always taught to look for the ‘squares.’

“The grid can pull you back hundreds of yards and make it difficult to stay still.”

Another said: “That on a large scale would be so scary. They would be constantly being pulled and hit by both sides of the waves.”

Waves around the world are getting stronger thanks to climate change, a report warns.

We’ve rounded up where to catch some of the world’s biggest waves.

A safer way to catch waves is in an inland surf lagoon – with the world’s first artificial inland lagoon found on the outskirts of Snowdonia

Or you can head to Bristol to try The Wave, a 180m high surf lake.

The waves appear like a chess board on the surface


The waves appear like a chess board on the surfacePhoto credit: YouTube/John Bern
If you wish to see them, it is best to do so from the safety of the country


If you wish to see them, it is best to do so from the safety of the countryPhoto credit: Shutterstock

Emma James

Emma James is a USTimesPost U.S. News Reporter based in London. His focus is on U.S. politics and the environment. He has covered climate change extensively, as well as healthcare and crime. Emma James joined USTimesPost in 2023 from the Daily Express and previously worked for Chemist and Druggist and the Jewish Chronicle. He is a graduate of Cambridge University. Languages: English. You can get in touch with me by emailing

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