Your Pool can Float!!!
An in-ground swimming pool may seem like a solid, unmovable structure, but it is actually sort of like a boat - it can float in the surrounding ground water. When the pool is empty, the ground water pressure can actually push the structure up out of the ground. This is one of the reasons why pools are filled with water year-round -- there needs to be roughly equal pressure on each side of the pool.
Many modern in-ground pools have a special hydrostatic valve near the main drain to guard against damage from ground water pressure. Basically, if the ground water pressure is great enough, it will push up on a small float, which opens the valve. When the valve opens, ground water flows in and equalizes the pressure.
Pool Trivia
