A hole dug into an aquifer will turn into a water well because the openings between the pieces of sand or gravel or shattered stone are so large that water molecules can easily flow.
The thickness of an aquifer from top to bottom varies with the nature of the earth. An aquifer can be only a few feet or inches thick, or it might be hundreds of feet thick. It might extend for miles without a break, like the Ogallala Aquifer which stretches from Nebraska to Texas. This aquifer is as large as California and contains as much water as Lake Huron. The aquifer might only be pond-sized, or it might stop abruptly because of some prehistoric geological upheaval. Generally an aquifer follows the topography of the land. Where the land dips, the aquifer does likewise.
From "Wells And Septic Systems"
by Max and Charlotte Alth