From the High Plains to the Big Bend, the Hill Country to the Piney Woods and places in between, Texas's natural beauty has always been part of its broad appeal.

But Texas also has a less familiar attraction: some 3,000 caves that make up a vast expanse of underground wonder.

Texas has more caves than any other state. For generations they have provided utility, historic fascination and scientific insight into life below and above the surface.

Underground Texas yields information for scientists and students in biology, geology, paleontology, anthropology, archaeology and speleology-the study of caves. In fact, more than 1,200 animal species have been found in Texas caves.

Several caves are National Natural Landmarks, but many are on private land. I co-sponsored the Endangered Species Recovery Act of 2007 to provide tax incentives to private landowners who voluntarily protect habitat of endangered or threatened species. This approach respects property rights of Texas landowners.

Texas has both "wild caves" and "show caves." Wild caves exist in their natural state, with no lighting or public paths. Honey Creek Cave is the longest cave in Texas, stretching more than 20 miles underneath Comal and Kendall counties.

Exploration of a wild cave may involve crawling through water, mud, insects and other distractions, or descending into total darkness on a rope. It has been compared to mountain climbing, "only backwards."

The state's seven show caves provide easier access, and welcome thousands of visitors every year. Most are in the Texas Hill Country, with several between Austin and San Antonio. The westernmost show cave is the Caverns of Sonora, about 170 miles northwest of San Antonio.

Show caves often feature translucent, icicle-shaped stalactites, stalagmites, crystalline walls and mirror-like pools. The remarkable formations in caves result from the continual dripping of mineral-rich water over millions of years, and the flow of underground streams.

Caves figured prominently in Texas history. Analysis of arrow points and other artifacts indicate some Native Americans lived in Texas caves, beginning thousands of years ago.

Comanche Indians once used Longhorn Cavern, near Burnet, for shelter. The cavern went on to become a gunpowder factory for the Confederacy and later a nightclub during the Roaring Twenties.

In an ironic conversion, Longhorn Cavern once even housed church services. The state of Texas bought Longhorn Cavern and opened it to the public as part of a state park in 1932.

Even without embellishment, Texas caves can offer a great adventure for young people-particularly in the summer, when they also provide an escape from summer heat.

When temperatures above ground soar into the 90s or higher, most caves have constant temperatures around 70.

There is no substitute for visiting caves in person and learning more about life under the ground of Texas. They're a little-noticed part of what makes our state great.

John Cornyn is a U.S. Senator for the state of Texas. Readers may contact him via telephone at (202) 224-2934.