Central Texas was Once Home to an Active Volcano
Pilot Knob is located seven miles south of Austin, near Bergstrom Air Force Base. It is home to a volcano that was active about 79.5 million years ago according to the University of Texas.
The article states that Pilot Knob is about two miles in diameter with Cottonmouth Creek at it's base. The area is described as an explosion crater where magma would would hit water and was quickly heated into steam. As the magma was cooled by the remaining water, it formed a hill that would eventually trap the magma into a pool of sorts.
The area is easily seen off Highway 183 just past Bergstrom Air Force Base but is on private land. So, if you make a weekend trip to check it out, get permission from the land owners, if possible, before going onto the land. Some people have taken pictures of the area as seen above and below.
Onlyinyourstate.com has some great pictures as well. However, they incorrectly state that the volcano is active. It is not. I would trust the work of UT Austin before this.
But for further, scientific details of Pilot Knob, read the article from The Walter Geology Library at UT Austin.
When I was introduced to this, I had no idea Texas had a volcano, active or not. It's just cool to think at one time, what we know as Texas, was an active area of (in my best Dr. Evil voice) liquid, hot mag-ma.