Mineral formations that look like icicles and grow slowly down from the ceilings of caves are called stalactites. They occur when rainwater and snowmelt seep through cracks in limestone. The slow, steady drip of water from a cave ceiling, enriched now with calcium carbonate, creates a small, tapering deposit of calcite. The stalactite begins as a hollow tube, conical in shape but sometimes straight (a “soda straw”). Eventually, after millions—countless—drops of water have slid down its ﬂanks and fallen to the cave ﬂoor, the tube closes, though it continues to grow. This creative force is secreted, hidden, in the darkness of the cave. Where the lime-rich drops hit the cave ﬂoor, a stalagmite can start to grow, building up, rising toward the stalactite above. If they come together, the formation they make is called a column. These creations have an inviting beauty and strangeness that compel humans to enter the cave’s darkness. At the same time, there is a sense that this is a world closing itself up, teeth, a cage, as if to say keep out, stay away, do not enter, do not leave, we are closing Earth.