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.…