On a small island off the west coast of France, you can witness some of the most remarkable ocean patterns on the planet. Isle of Rhé is renowned for its pristine beaches and pleasant weather and draws tourists from all over Europe. The island is also perfectly situated to witness an ocean phenomenon known as a cross sea.

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A cross sea looks beautiful from afar but is actually very dangerous. The pattern occurs when two wave systems cross while traveling at slanting angles to each other. The waves from the older weather system continue on until they dissipate, and the ocean becomes a swirling, dangerous place for swimmers and boats.

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A cross sea can also sometimes produce very tall waves. Many ship accidents have been blamed on encountering cross sea conditions.

Photo Credit: NOAA

Cross seas can occur in coastal places around the globe, so if you ever happen to see one, take a photo, but stay out of the water!

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h/t: Gizmodo