A cave mouth is a door to mystery and beauty, the entryway to a mineral world of water and moving air that, over time, has become a sacred place. A womb of Earth. Many cave walls were once painted with animals and the history of different peoples. In far deeper caves, Earth has painted its own history. Some caves developed during the life-nourishing eruptions of the planet: lava tubes, where magma runs underground and leaves empty tunnels behind. Some are tectonic, created by quaking movements of the planet. And there are long-lived caves of ice. The caves most widely known in the United States, however, are those created by dissolution and erosion in karst landscapes. “The ﬁnest workers in stone are not copper and steel tools,” wrote Henry David Thoreau, “but the gentle touches of air and water working at their leisure with a liberal allowance of time.” Karst caves include passageways and rooms with mineral deposits in the form of stalactites and stalagmites, soda straws, and draperylike ribbons, all built up by trickles of calcite-bearing water. Patricia Hampl describes this water in Romantic Education as “running steadily, timelessly, making its slow, hypnotic mark on the stone, on the ear, on the brain.” Caves have their own ecosystems and many animals and insects depend on them. Not just hibernating bears but resident blind crayﬁsh and endangered cave ﬁsh. Many caves harbor bats and indigenous beetles and salamanders. Carlsbad Caverns in New Mexico shelters crystal formations in cathedral-like rooms. The stable temperature in caves near San Antonio preserves bat guano, once used to make gunpowder. Mammoth Cave in Kentucky is the world’s longest cave system, with 350 miles of chambers and passages.