Texas has three distinct landforms:
Most of the Texan caves are located in the center, especially in the Balcones Escarpment the edge of the Edwards Plateau near Austin and San Antonio. Texas has eight show caves, six of them are located between San Antonio and Austin within day-trip range. More caves, springs and dolines are located to the northwest of Austin, including one show cave. The area around Austin is called Hill Country. Only the Cavern of Sonora lies 275 km west of San Antonio.
The area northeast of San Antonio is actually quite famous. The underground karst water or aquifer is known under the name Edwards Aquifer. It is a natural resource which serves the diverse needs of almost two million users in south central Texas. The ground water is used for agricultural, industrial, recreational, and domestic purposes. It is especially important as the area is semi-arid and water is fundamental to any human development. However, as the recharge in a semi-arid area is quite low it is also in danger. Water which is removed beyond the Aquifer's capacity depletes the aquifer and might finally result in the collapse of the water supply. Currently the goal is to develop a rational and sustainable management of this natural resource.
Four show caves of Texas offer a free t-shirt with the print Texas Cave Trail as a sort of combo bonus. The caves are Natural Bridge Caverns, Caverns of Sonora, Cave Without a Name, and Inner Space Cavern. If you visit one of those caves you can ask for a free passport, which is stamped during your visit to that cave. If you visit all four caves during one year, you can exchange your completed passport for your free tee shirt.