“The name, Devil’s Sinkhole, aptly connotes the dark depths and other-worldliness of a vast vertical chasm in the limestone bedrock on the far western reaches of the Edwards Plateau,” states the website Texas Beyond History.

A registered National Natural Landmark since 1985, the sinkhole plunges a remarkable 400 feet and has a 40-foot by 60-foot opening. It is the largest single-chambered cavern and the third-deepest in the state. “Part of a vast karst system of underground caves in the porous Edwards Plateau, the sinkhole was created when an underground solution cavity collapsed.”

This is archeological evidence that the sinkhole was once considered sacred by Native Americans, as locals have found stalactites, arrowheads, and other treasures from the area. Evidence from around the site indicates that the sinkhole may have been used for burying the dead, as “Native Americans considered sinkholes as well as caves, crevices, springs, and seeps to be avenues to the underworld and the earth gods…In much later times, the sinkhole was a gathering place for cowboys and cavers, who etched their names and other graffiti into the limestone walls.”

Today the sinkhole is a summer home to one to four million Mexican free-tailed bats. (The same bats that call Austin’s Congress Avenue Bridge home.) At sundown, visitors can begin to see a few bats emerge, until they become a black swirling mass of millions flying into the evening sky.

Evening flight tours are offered Wednesday through Saturday during the summer to witness the bat emergence. Access to the natural area is limited to tours, and reservations are required through the Devil’s Sinkhole Society.