IF you see grid-like waves in the sea next time you are at the beach, you should get out of the water right away.
Called a cross sea, they are so dangerous that they have even caused shipwrecks over the years.
Wikipedia/Michel GriffonA “cross sea” is very dangerous to swim in – and has even caused shipwrecks[/caption]
YouTube/John BernThey are also known as square waves[/caption]
Also known as square waves, they may look interesting and are usually found out in the open waters although can be spotted nearer the coastline.
Places they have been spotted are in France, including on the Île de Ré near La Rochelle as well as near Lisbon in Portugal.
Cross seas are caused by two separate weather systems collide.
They are very similar in rip tides which can be difficult to get out of due to the two different directions of waves.
To spot them, they appear on top of the water is a chequerboard effect on the surface.
According to the European Space Agency, they have even caused a number of shipwrecks over the years due to the different directions of the waves causing ships to roll.
A German research icebreaker vessel called RV Polarstern reported a dangerously heavy rolling incident, which was said to be caused by a cross sea.
If you do want to see the cross waves, it is best to spot them from land to see the full effect while staying safe.
People on social media shared their experiences of square waves – and how dangerous they are.
One person on YouTube said: “Growing up in Hawaii, we were always taught to keep an eye out for the ‘squares’.
“The grid can pull you back and fourth for hundreds of feet and make it hard to stay calm.”
Another said: “This on a big scale would be so scary. You would constantly get dragged and hit from both sides of the waves.”
Waves around the world are getting stronger thanks to climate change, a report has warned.
We’ve rounded up where to spot some of the biggest waves in the world.
A safer way to catch some waves is at an inland surfing lagoon – with the world’s first inland artificial lagoon found on the outskirts of Snowdonia
Or you can head to Bristol to try out The Wave, a 590ft (180m) surfing lake.
YouTube/John BernThe waves appear like a chequerboard on the surfae[/caption]
ShutterstockIf you want to spot them it is best to do it from the safety of the land[/caption]