Summer beach days are all about fun in the sun, but amid the laughter and splashing, nature can turn deadly in an instant. While many know about rip currents and shifting tides, few recognize the threat of square waves—a mesmerizing yet dangerous ocean phenomenon.

Also known as cross seas, these waves form when two or more swells collide at right angles, creating a checkerboard-like pattern on the surface. According to the European Space Agency (ESA), this happens “when a windsea and a swell, or two swell systems, coexist.” A 2004 study even revealed that a large number of ship accidents occurred in these crossing sea conditions.

Square waves can rise up to 10 feet high, making navigation treacherous for boats and swimming nearly impossible. They often appear near peninsulas where two bodies of water meet—such as the Isle of Ré in France and Cape Reinga in New Zealand, where the Tasman Sea and Pacific Ocean collide.

“They may look breathtaking, but square waves demand respect. Always admire them from a safe distance.”