You’ve probably created one without realizing it.
Slosh the water back and forth in a bathtub and, under the right conditions, the water begins oscillating from one end to the other. Scale that phenomenon up to a lake, bay or harbor, however, and the result can become legitimately dangerous.
It’s called a seiche — pronounced “saysh” — a type of standing wave. And a viral video illustrating the phenomenon comes with a simple warning right in its title: “If the waves do this, get out of the water now.” Check it out below.
According to NOAA, “A seiche is a standing wave oscillating in a body of water.” Unlike a normal ocean swell traveling toward shore, a standing wave oscillates within an enclosed or semi-enclosed body of water, somewhat like a seesaw. Strong winds and rapidly changing atmospheric pressure can push water toward one side before it rebounds when conditions change. Earthquakes, tsunamis and severe storm fronts can also trigger them.
And they don’t necessarily stop quickly.
NOAA says the water can continue to “oscillate back and forth for hours or even days.” The agency points to a terrifying example from Lake Erie in 1844, when a 22-foot seiche breached a 14-foot seawall, killing 78 people.
The danger isn’t just theoretical.
Related: Watch: Massive Dam Opens Gates After 5 Years, Unleashing World’s Biggest Standing Wave
One commenter on the video wrote about a real-life encounter with the phenomenon:
“I’m a trained lifeguard.
“I once jumped into something like this to save someone who fell into the water off his boat. I had a lifejacket on, the guy I rescued did not. While the crests and valleys were a bit lower than this, it was still excruciatingly difficult to keep our heads out of water.
“It was probably the most difficult thing I’ve ever done, trying to do a rescue in this situation.”
Technically, not every stationary wave is a seiche — standing waves can also form through powerful opposing currents and hydraulic effects. But the visual warning is similar: water behaving strangely, oscillating or appearing to remain fixed while enormous volumes of water move through it, deserves respect.
A few inches sloshing around your bathtub is amusing.
Thousands or millions of gallons doing the same thing is something else entirely.
Related: Watch: 20-Foot Rogue Wave Capsizes Yacht in the Middle of Pacific Ocean
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