In-ground pools look to be solid and immovable, yet they are more like a boat. The pool might float up out of the earth if the ground water level rises. As a result, most in-ground pools have a hydrostatic pressure release valve that equalises the pressure inside the pool with the surrounding soil and ground water, preventing a floating motion that might harm the basin.
A drain is usually found at the bottom of every in-ground pool's basin. This drain is for draining the pool and drawing pool water through the filter. When the stopper is pulled in a bathtub, a vortex forms at the place where the water drains.