It percolates into the ground. It descends through the soil column above the water table by gravity until it reaches the water table. In dense soils, it may cause a localized groundwater mound if the leak is significant. Then it travels underground with the groundwater, eventually flowing out of a spring or into a stream, river, or ocean; or it may be pumped out of the ground via a private or public water well. If you roll a ball down the land surface, that is the direction in which groundwater is most likely to flow.
Although I have never investigated a residential property with a leaking pool, I have measured this effect in the groundwater depths surrounding multiple car washes with leaking settling basins and recycled-water tanks.
A swimming pool that is installed in the ground should not leak. They are purposefully designed with a drain that redirects drained water back into the filter system.
ALSO SEE: Sulfuric Acid for Cesspools
As with all other water, this water is filtered and returned to the pool. In the event that the drain fails, an inground pool may lift from the ground.
As earlier stated, an above ground pool leak should be obvious as to where it is going, on or into the ground, but it does not always, in fact, does not usually, join and follow the water table; instead, it washes the ground out from under the pool if it is going under it.
In a “above ground” pool, the walls are not attached to or sunk into the ground. They are simply seated on the ground.
The wall is suspended in a channel above the ground, but they are only touching the ground surface. Because the walls are not sealed at all, it is obvious where the water is going if / when the liner of an above ground leaks.
At the very least, the ground will be muddy. When an above ground pool liner develops a hole, it rips a large hole in the liner and the wall collapses, spilling water everywhere.
It may still hold if there is a small hole in the liner on the floor, but this is usually the case if the hole is in a wall.