In March 1971, two airmen station-ed at Bergstrom Airforce Base were walking along Barton Creek. Spring foliage was just beginning to color the creek’s banks, but through the Greenbelt’s budding leaves, the pair spied a crack in an adjacent cliff face. A steady rush of cool air pulsed through the crevice, indicating that it extended far deeper than the 6-inch-wide fissure let on. Eager to see where the hole led, the men began to dig, discovering an underground expanse that’s become the stuff of spelunking legend.

Nearly five decades later, Airmen’s Cave is considered one of the most mysterious (and perilous) caves in Texas. Just over 2 miles long, each of its sections present a litany of physical challenges. For example, to get through the entrance (known as the “birthing canal”), cavers must inch forward—guided only by the dim light of a headlamp—through a claustrophobia-inducing channel that’s just big enough to fit a full-grown adult. From there, the maze-like “Sherwood Forest” requires you to crawl on all fours while dodging stalactites hanging from its 2-foot ceiling. Later, the “One-Legged Man” stretch forces explorers to contort their bodies and hop on one foot through its cramped corridors.

There’s also the “Aggie Art Gallery” located halfway through: a room littered with clay sculptures of dragons, monsters, and, ahem, phallic figures created by past cavers. Though most turn back from there, some venture on in search of the mythical “Geode Room,” which many amateurs claim (though none have proven) is covered wall-to-wall in glistening crystals.

Due to several emergency rescues that have taken place over the decades, the City of Austin has hired a designated cave manager to watch over Airmen’s Cave—making them the gatekeeper to one of the city’s most breathtaking and mystical secrets.