You might have been taught to make a swift exit from the ocean if you spot a sinister-looking sea creature, but what about when you see square waves?
Although the majority of us are well versed in the basics of staying safe in the water, not a lot of people are aware of the dangers of this bizarre weather phenomenon.
Square waves might be a sight to behold, but they can also be deadly if you find yourself caught up in them.
Although most of us imagine ocean waves calmly rolling into shore in straight lines, they can present themselves in much more unpredictable forms too.
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Even the most experienced swimmers could end up in peril if they stay in the water in certain conditions - but you seriously need to skedaddle if you see a chessboard-like pattern on the water.
Oceanographers explain that when two particularly strong weather patterns and opposing swells collide, they can force waves to form at different angles, creating the 'square' design.
It might be interesting to look at, but you need to head for dry land ASAP if you spot this happening while you are having a dip in the sea.
These dangerous waves can form within a matter of minutes and reach heights of up to 10ft, but can disappear just as fast as they rock up.
According to Surfer Today, this ocean phenomenon is typically associated with strong and localised rip tides, which is a strong offshore current that is caused when the tide pulls water along the beach.
These can easily overpower swimmers and you could be dragged off by the waves and below the surface if you're paddling in a section of square waves.
They can even get the better of boats.
Square waves caused a huge percentage of shipwrecks between 1995 and 1999, the European Space Agency (ESA) said.
These cross swells are really, really difficult to swim out of - so if you want to avoid getting sucked into a riptide, you need to evacuate the ocean immediately as soon as you spot them.
But if you do find yourself in this situation, experts have some advice which could save your life.
Mathew Giachetti, of the United States Lifesaving Association (USLA), previously told Islands.com: "If caught in a square wave, the most important thing to do is get out of the water immediately, as they are extremely dangerous.
"Stay calm, try to float on your back, and swim parallel to the shore to escape the wave's force. Then carefully make your way back to land as soon as possible; if you can't reach the shore quickly, signal for help."
Square waves often occur along coastal areas, such as the French island of Île de Ré, and thousands of tourists flock there to see the phenomenon with their own eyes.
They've also been known to turn up in the South Pacific and the Atlantic Ocean as well as other spots, so keep an eye out for them no matter where you are in the world.
Olivia Burke