Summers are synonymous with trips to the beach, wading in the sea, and soaking up the sun. It’s a season of fun and relaxation, which is why beaches are often crowded. But while enjoying the water, it’s crucial to stay vigilant. Even seemingly calm seas can hide hidden dangers. Beyond the familiar tides and rip currents, one phenomenon stands out for its beauty and risk: square waves. These mesmerizing ocean patterns can be stunning to watch—but they are also extremely hazardous.

Square waves, also known as cross seas, occur when two or more swells intersect, forming a grid-like or checkerboard pattern on the water’s surface. According to the European Space Agency (ESA), “The conditions are quite common in the ocean and occur when a windsea and a swell, or two swell systems, coexist.” Research from 2004 also highlights the danger, showing that a significant percentage of ship accidents happened in crossing sea states. While rare, encountering square waves can be life-threatening for both swimmers and boaters.

These crossed seas can generate swells as high as 10 feet, creating unpredictable patterns in the wind and waves. For swimmers, navigating these waters becomes nearly impossible, and boaters risk capsizing or losing control. If one happens upon a square wave, it is safest to avoid entering the water entirely. Instead, beachgoers should enjoy the view from the sand, waiting for calmer conditions before venturing into the sea.

Square waves are most commonly observed at peninsulas or locations where large water bodies converge. As swells approach from different angles and often refract around the landmass, the unique grid-like wave patterns appear. Notable locations include the Isle of Re off the coast of La Rochelle, France, and Cape Reinga at the northern tip of New Zealand, where the Tasman Sea and the Pacific Ocean meet. These natural phenomena have made these spots popular tourist destinations, drawing thousands eager to witness the rare and stunning spectacle.

While these waves are beautiful, safety must come first. Observing from a distance is key, as venturing into the water when swells intersect at varying angles can be extremely dangerous. Interestingly, square waves are not limited to deep waters; they can sometimes be seen in shallower areas, such as off Tel Aviv, Israel. Scientists explain that square waves are an example of the Kadomtsev-Petviashvili equation in action, which describes nonlinear wave motion and shows how different weather systems interact.

Ultimately, square waves are a striking reminder of the ocean’s beauty and power. While they provide a rare and captivating sight, they are not to be underestimated. Whether visiting the Isle of Re, Cape Reinga, or other locations where cross seas form, it’s vital to admire these natural wonders safely—from the shoreline rather than in the water. Enjoy the view, take photos, but always respect the ocean’s unpredictable and potentially deadly force.