This popular attraction is a relatively new discovery. In March of 1960, four college students from nearby St. Mary’s University were given permission to explore the area underneath a 60-foot (18-meter) limestone bridge. They eventually discovered these caverns, which are the largest known commercial ones in Texas. The Natural Bridge Caverns boast spectacular underground formations, including stalagmites, flowstones and stalactites.
You can choose from several different tours — which one you will want to take will depend on your interest level, walking abilities and comfort with being in caves. You could, for example, go on a standard Discovery tour, which will take you through the main caves. If you’re more adventurous, you can rappel to less developed areas of the caverns where there is no commercial lighting. 
Interesting to know next time I am i n the US and am in the Texas area. Thanks again