Next time you're sitting in the tub, try the following experiment. Stand up quickly, being careful not to fall over. The water will slosh over to fill in the place where your body was, and then slosh back in the other direction, and so on. You have created what is called a standing wave inside your tub, or a wave that reverberates between two end-points.
The same thing happens in any body of water that has walls to it, such as a lake. Instead of a visible, conspicuous wave, however, lakes can have a completely hidden wave that travels under the surface. Here's how.
In a lake, cold water sinks to the bottom, forming a distinct layer underneath the warmer water on top. Wind blows across the top layer. This causes more of that warm layer to build up on one end of the lake, pushing the cold layer underneath in the opposite direction. When the wind stops the warm water sloshes back, pushing the cold water in the other direction. There are now two sloshing waves that balance each other out. Unlike your tub, however, because they balance out the surface remains calm.
That hidden sloshing can be enormously powerful. Sometimes the slosh-wave can be up to three hundred feet deep. It can scour the bottom of lakes and cause objects resting there to be flung up toward the shores.
Is this what people who claim to have seen lake monsters are really seeing old tree trunks that suddenly emerge from a calm surface and mysteriously sink again.