The Dangerous Ocean Phenomenon You've Never Heard Of

Everyone knows the ocean is dangerous, and in almost too many ways to count. From booming storms tossing ships around, massive tsunami racing from fault lines towards the shore, or whirlpools swirling like the flush of a giant toilet bowl, the ocean has claimed countless lives over the course of human history. It's no wonder that stories of sea monsters have circulated across the globe to serve as cautionary tales for anyone willing to venture out towards the bright, blue, endless horizon.

And those are just the most cinematic dangers. There are also plenty of other, more subtle ways the ocean can get you. The undertow, for instance, is a downward sucking force that happens when water rushes up to a beach and then back out to the ocean, and it's been known to pull small children under. Rip currents are similar, but are actually waist-to-chest-high waves that don't break, making their way to shore and then flowing quickly back out to the surf line. Rip tide, as the name says, is a current caused by tidal flow to or from the shore, especially along the edge of an inlet, and the sudden rush of water can be deadly.

But there's one dangerous ocean phenomenon you've probably never heard of even though it's actually spectacularly weird to behold: cross seas.

A cross sea occurs when wind blows two opposing sets of wave forms towards each other at an oblique angle — diagonal, or 45-degree. The waves collide, but do not cancel each other out, as they would if they hit head-on. Instead, they pass through. According to the University at Buffalo, the mathematical principle on display is called the Kadomtsev–Petviashvili equation, which is a fun bit of trivia that should bring you comfort as you're sucked to the bottom of the sea.

That equation is what explains the distinctive square waves that make cross seas look like a weird ocean quilt. The pattern is so geometrically pristine that it captures the eye of both scientists and tourists with its unusual appearance.

Seeing it, though, can be tricky. The phenomenon typically crops up and dies out in a matter of seconds, because the energy of cross sea systems weakens rather quickly. Smaller wind patterns can also interfere with their strength.

There are spots on the earth, though, where conditions are rife for cross sea creation on a regular basis. At the Isle of Rhé off the west coast of France, for instance, visitors come by the thousands to view cross sea's perpendicular lines from the location's lighthouse. Tel Aviv, Israel, is another common spot for observing them, while Lisbon, Portugal, has also been known to afford good views of cross seas.

Like any other ocean phenomenon, cross seas ought to be respected, and avoided if at all possible. Unlike rip currents, though, which cause approximately 100 deaths to swimmers and surfers each year, per World Atlas, cross seas are easily visible and can even be spotted at a distance. And luckily, if you're just standing along the edge of some wet sand with water around your ankles, cross sea waves aren't about to rush up and nab you. Even for the most anxious among us, that has to be a comfort.

In truth, cross seas are more dangerous to boats than to swimmers. According to The Guardian, if a boat gets caught precisely at the intersection of two waves, it can be rocked back and forth and take on water. This is why a disproportionately large number of accidents occur from the waves of cross seas, doubtlessly combined with the inexperience of boaters who may not be used to seeing such a rare phenomenon.

If you do happen to be swimming, however, and find yourself caught in a cross sea, conserve your energy and do your best not to panic; just wait for the wave to pass. Barring that common sense advice, the simplest solution is often the best: if you're worried about getting caught in some current or tidal flow, then just stay in your beach chair with a beer and stay out of the water entirely.

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