It wasn't until 2017 that European spelunkers entered and named the Brgevčeva cave system in Slovenia. Inside, the explorers found a narrow corridor that opens onto a giant hall filled with baroque stalagmites and stalactites, and in one passageway they discovered 15,000-year-old bones of bison and horses. Cavers strive to step only on established routes to preserve pristine subterranean worlds such as this one.