Square waves, though rarely seen in ocean settings, are highly dangerous and pose serious risks.

The sight of ocean waves crashing against rugged, rocky shores is mesmerizing, but square waves, or cross-sea waves, offer an experience unlike any other. These surreal and captivating waves have been the cause of numerous boating accidents.

Keep reading to dive deeper into the mysteries of this extraordinary yet hazardous natural phenomenon.

How Waves Are Created

Before diving into the science of cross-seas, it's important to first understand how ocean waves form. According to the National Oceanic and Atmospheric Administration, "Waves are the result of energy traveling through water, making it move in circular patterns. However, the water itself doesn't move in waves."

Wind is the primary force behind the formation of ocean waves, distinct from those caused by earthquakes, hurricanes, landslides, or volcanic eruptions. As the wind blows, it transfers energy to the water’s surface, creating continuous disruptions that lead to wave crests.

The moon and the sun can also generate waves, known as tidal waves. It's important to note, however, that a tidal wave is not the same as a tsunami.

How Cross-Sea Waves Develop

Normally, waves move parallel to the coastline, but cross-seas are a different phenomenon. They occur when two opposing swells meet and collide. This happens when winds from two different weather systems — one older, the other newer — converge. The newer system's waves travel at an angle to the older system's waves, which continue in their original direction despite the change in wind.

The result is a patchwork-like pattern of squares on the ocean's surface, with the waves moving in a grid formation.

Cross-seas are relatively rare but are more common along certain coastal regions. One place where they appear with remarkable frequency is along the west coast of France. The unique geology of the Isle of Rhé creates the ideal conditions for their formation.

This site has become a popular tourist attraction, drawing thousands to the island's lighthouse each year to observe the phenomenon from a safe distance. It's a bit curious that intersecting waves would captivate so many, but when was the last time you saw nature craft squares on water?

Navigating through a squared sea can be challenging for both boaters and swimmers. The danger of cross-seas lies in their ability to create powerful rip currents and towering waves, some reaching as high as 10 feet (3 meters) — more than enough to overwhelm a large boat. A study found that "a large percentage of ship accidents occurred in crossing sea states."

In shallow waters, such as those near the Isle of Rhé or Tel Aviv, Israel, you can often spot two opposing swells. According to scientists, cross-seas are an example of the Kadomstev-Petviashvili equation in action. This formula describes nonlinear wave motion and is frequently used to explain the interaction between different weather systems.

What To Do If You Encounter Square Waves

If you spot square waves, it's crucial to get out of the water right away, as they pose a real threat. However, spotting the pattern made by the two opposing swells while swimming might not be easy, so your best strategy is to avoid venturing too far out into the water.