Sous les pav, Edwards Aquifer:

The Edwards Aquifer is a major resource for the area, and the focal point of much political debate.

An aquifer is a bunch of underground water in porous rock. The Edwards Aquifer churns out the water for Barton Springs. If there's too much development over it, the pollution seeping in may make Barton Springs unsafe for swimming.

Aquifers are important and impressive.

A resource page on the Edwards Aquifer.