Square Waves: The Beautiful but Deadly Ocean Phenomenon

What Are Square Waves?

The Science Behind Their Formation

If you've ever seen a checkerboard pattern on the surface of the ocean, you've witnessed a rare and mesmerizing phenomenon known as square waves. At first glance, they might seem like a trick of the eye—an underwater grid shaping the sea. But in reality, these formations result from complex interactions between different wave systems.

Square waves, also called cross seas, occur when two opposing swells—waves generated by different weather systems—collide at oblique angles. Instead of blending smoothly, the waves create a crisscross pattern, forming what appears to be a grid on the water's surface.

This phenomenon happens due to:

• Wave refraction – the bending of waves as they approach different depths.
• Wave diffraction – the redirection of waves after they hit obstacles like cliffs or breakwaters.

In some cases, square waves can rise up to 10 feet (3 meters) high, creating strong and unpredictable currents. While they might look stunning from above, these unique waves pose serious dangers to swimmers, surfers, and even boats.

Where They Can Be Found in the World

Square waves are relatively rare, but they can appear in several locations around the world. One of the most famous spots for observing them is Île de Ré, an island off the western coast of France. Tourists visiting the island often climb lighthouses or cliffs to witness the spectacular pattern.

However, square waves have also been spotted in:

• The South Pacific Ocean
• The Atlantic Ocean
• Lisbon, Portugal (between two breakwaters)
• Whangamatā Beach, New Zealand
• Mustikkamaa Island, Finland (on a smaller scale)

Despite their rarity, square waves can appear and disappear within minutes, making them difficult to predict. This unpredictability is part of what makes them so hazardous.

Why Are Square Waves So Dangerous?

The Physics Behind Their Powerful Currents

Square waves are not just an optical marvel; they are also one of the most dangerous oceanic conditions. Their power lies in the cross currents they generate.

Unlike traditional waves that push water in a single direction, square waves create competing currents. This results in:

• Unpredictable movement – The water flows in multiple directions simultaneously, making it difficult for swimmers and boats to navigate.
• Strong rip currents – These can pull even experienced swimmers out to sea in a matter of seconds.
• Instability for boats – Vessels caught in square waves can experience violent rocking, increasing the risk of capsizing.

Since square waves often form in open waters, they pose a significant risk to ships and smaller boats. Even on a seemingly calm day, the sudden emergence of cross seas can be treacherous.

The Connection Between Cross Seas and Shipwrecks

The European Space Agency (ESA) has reported that cross seas have been responsible for a high percentage of ship accidents. Between 1995 and 1999, numerous maritime incidents occurred in areas where cross seas were detected.

Ships navigating through these waters face several hazards:

• Extreme rolling and tilting – Ships can become unstable, leading to cargo displacement or structural damage.
• Engine failure due to water intake – The choppy, multidirectional motion increases the risk of flooding in engine compartments.
• Crew disorientation and injuries – The unpredictable movement makes it harder for sailors to maintain control.

One notable example involved the German research icebreaker RV Polarstern, which recorded a heavy rolling event caused by square waves during a scientific expedition.

To mitigate these dangers, modern satellite technology—such as Synthetic-Aperture Radar (SAR)—is now used to detect potentially hazardous cross-sea scenarios, even in poor weather conditions. However, for casual swimmers, surfers, or beachgoers, the best protection remains awareness and avoidance.

How Do Square Waves Form?

The Role of Opposing Swells

Square waves form due to a unique oceanic interaction known as a cross sea, which happens when two wave systems moving in different directions collide at an angle. Instead of merging seamlessly, these opposing swells create a grid-like pattern on the water’s surface.

This phenomenon typically occurs when:

• A distant storm system generates swells that travel across the ocean.
• A second, local weather system creates waves in a different direction.
• The two swells intersect at an oblique angle, forming the characteristic crisscross pattern.

The interaction between these swells creates turbulent water conditions, making it difficult for swimmers, boats, and even large ships to navigate safely.

Why They Occur Near the Isle of Ré

One of the most famous locations for witnessing square waves is Île de Ré, a small island off the western coast of France. The region’s geographical and meteorological conditions make it a hotspot for this rare phenomenon.

Square waves are common in Île de Ré because:

• The island sits at the meeting point of two major seas—the Atlantic Ocean and the Bay of Biscay.
• Seasonal weather systems create opposing swells that frequently intersect.
• The surrounding coastal features and underwater topography influence wave refraction, enhancing the cross-sea effect.

Tourists flock to Île de Ré to capture this stunning natural spectacle from lighthouses and cliffs. However, local authorities warn visitors to stay out of the water when square waves appear, as they can create powerful rip currents and treacherous conditions for swimmers and boats alike.

What Happens If You Get Caught in Square Waves?

The Risks of Being in the Water When They Appear

If you’re swimming, surfing, or boating and suddenly find yourself in an area with square waves, you could be in serious danger. Unlike normal waves that move in a single direction, square waves create unpredictable and powerful currents that pull people and objects in multiple directions at once.

The key risks include:

• Being pulled out to sea – The chaotic currents make it extremely difficult to swim back to shore.
• Sudden waves crashing from different angles – Unlike traditional rip currents, square waves don’t follow a single flow, making escape even harder.
• Difficulties for boats – The multidirectional swells make it easy for smaller boats to capsize, while larger ships may experience dangerous rolling and instability.

If you’re in the water when square waves appear, the best thing to do is get out immediately. Even experienced swimmers struggle against the unpredictable currents, and surfers risk being thrown off their boards in unexpected directions.

Real Cases of Swimmers and Boats in Trouble

While square waves are visually fascinating, they have caused several maritime and swimming accidents over the years.

• In France, near Île de Ré, lifeguards regularly warn beach goers to stay out of the water when square waves form. Despite the warnings, some tourists unfamiliar with the phenomenon have found themselves caught in dangerous cross-sea conditions, requiring emergency rescues.
• A 2017 incident in Portugal saw a group of surfers struggle against the force of an unexpected cross-sea current. Even with experience, they had difficulty escaping the chaotic water patterns.
• According to the European Space Agency (ESA), square waves have been linked to numerous shipwrecks and maritime accidents, particularly between 1995 and 1999, when many cargo ships and smaller vessels were lost due to cross-sea instability.

In some cases, square waves disappear within minutes, making it hard to predict their occurrence. That’s why staying informed and recognizing the warning signs is essential for beach safety.

How to Stay Safe If You Spot Square Waves

Signs to Watch For

The best way to stay safe from square waves is to recognize them before entering the water. While they may appear mesmerizing from above, their presence signals a dangerous cross-sea condition.

Key signs to watch for include:

• A checkerboard or grid-like pattern on the water’s surface.
• Waves moving in two different directions instead of the usual parallel motion.
• Strong, unpredictable currents that seem to push and pull in multiple directions.
• Rapid changes in wave behavior, where calm water suddenly becomes choppy and unstable.

You can often spot square waves from a high vantage point, such as a cliff, lighthouse, or drone footage. If you notice these patterns, avoid entering the water and warn others nearby.

What to Do (and NOT Do) If You’re Near Them

If you find yourself in an area where square waves are forming, act immediately to ensure your safety.

✅ Do:

• Get out of the water immediately if you notice the pattern forming.
• Alert others who may not be aware of the danger.
• Avoid boating or surfing in cross-sea conditions.
• Check local weather and ocean conditions before entering unfamiliar waters.

❌ Don’t:

• Try to swim through the currents – they are too strong and unpredictable.
• Panic – staying calm and floating may help conserve energy if you’re caught.
• Underestimate their power – even small cross-seas can be dangerous.

For beach goers and swimmers, awareness is the best defense. If local authorities or warning signs indicate cross-seas, follow the guidance and stay onshore.

Final Thoughts: Square Waves—A Natural Wonder or a Hidden Danger?

The Balance Between Beauty and Risk

Square waves are undeniably one of the most fascinating oceanic phenomena. Their mesmerizing checkerboard pattern makes them a stunning sight, drawing in photographers, tourists, and ocean lovers. However, beneath their beauty lies a serious danger.

Unlike regular waves, square waves create unpredictable and powerful crosscurrents, making swimming, surfing, and even boating hazardous. What looks like a peaceful natural wonder from above can turn into a deadly trap for those caught in its grasp.

This duality—stunning to witness but dangerous to experience—makes square waves both a marvel and a menace.

Why More Awareness Is Needed

Despite their deadly potential, many people are unaware of the risks posed by square waves. Tourists visiting areas like Île de Ré often admire the phenomenon but may not realize the danger of entering the water.

Increasing awareness is crucial because:

• Many drownings and maritime accidents have been linked to cross-seas.
• Square waves can appear suddenly and disappear within minutes, making them hard to predict.
• Warning signs may not always be present, especially in remote or lesser-known locations.

By educating beachgoers, swimmers, and boaters about how to recognize and avoid square waves, we can help prevent unnecessary tragedies.

Next time you’re at the coast and spot the telltale grid-like pattern on the water, admire it from a safe distance—but stay out of the water. In the ocean, knowledge is survival.