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Today, if you've got somewhere to go, you might hop in a car or even take a plane. But for centuries, a boat was your fastest way of getting around. Except when it suddenly wasn't — for thousands of years, sailors have been telling stories of a mysterious phenomenon called dead water that could stop a ship in its tracks.

We now know that dead water is just, well, water, and after all these years, scientists finally understand why it used to mess with sailors — and how it might still affect swimmers today. Fridtjof Nansen, a Norweigian sailor and future Nobel Laureate, coined the phrase “dead water” back in 1893, but he wasn't the first to get stuck in it. Stories of ships moving under full power suddenly slowing or even totally stopping date back to at least 600 B.

C. E. Sailors over the years have tried all sorts of ways to escape, from pouring oil in front of the bow to shooting at the water and even banishing monks from their ships.

But, most of the time, if you're caught in dead water, you've just got to wait it out. After this had happened many times, people started to notice that dead water was more common where it was cold. For instance, Nansen's ship, Fram, was sailing north of Siberia when it got stuck.

This incident helped point to what was really going on: Sailors were getting stuck in places where a layer of cold freshwater was floating on top of the salty sea. Like, maybe meltwater from a nearby glacier. The two types of water act almost like totally separate fluids, and the boundary between them is a lot like the one between the water and the air.

And that can make things complicated. In 1904, the oceanographer V. Walfrid Ekman showed mathematically that when a boat runs into this two-layer system, it creates massive waves.

But those waves aren't at the surface where water meets air, they're at the boundary between saltwater and freshwater. Which is why, to a sailor, the surface of the water looks perfectly calm. Creating these waves takes a lot more energy than just producing the normal wakes you're used to seeing around boats — especially when the boat is moving slowly, close to the speed of the waves themselves.

Those underwater waves will steal the boat's momentum and slow it down in the water. And as long as those waves are there, the boat's not going anywhere — at least not fast. Ships can only escape once those waves dissipate, which usually happens when they run into the boat.

Fortunately, if you're traveling much faster than the underwater waves, this effect is pretty irrelevant. You'll just outrun the problem in the first place. So most modern ships don't have to worry.

But there is another kind of slow-moving vessel that might still need to worry about dead water: you. Among open-water swimmers, there are surprising cases of well-trained athletes drowning of exhaustion in perfectly calm water, and some researchers wondered if dead water could have anything to do with it. In a 2009 study, scientists found that swimmers moving through water with a thin, cool upper layer lost up to 40% of their motion.

There's still work being done in this area, but it could help explain why even strong swimmers can sometimes get trapped in calm water. So it's something to keep in mind if you go for a swim and feel water layers of different temperatures. We asked an expert and, if you find yourself trapped in such a situation, your best bet is probably to stay calm and use a slow, shallow stroke.

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