An aquifer is a layer of rock or sand that holds an underground lake.
Water comes up to the surface through springs, and is often pumped for drinking water.
The Edward's Aquifer is made up of a layer of limestone with a bunch of holes and caves, sort of like swiss cheese rock, with water running all through it. The holey-rock is called Karst. The Edwards Aquifer is under Central Texas and runs along the Balcones Escarpment between Austin and Uvalde. It's under Uvalde, Medina, Bexar, Comal and Hays counties. The Edwards Aquifer supplies drinking water for 1.5 million people, including San Antonio up through Austin.