Final answer:
Stalactites are the long, slender speleothems that hang from the ceiling of a cavern, formed from calcium carbonate deposits.
Step-by-step explanation:
The long, slender speleothems that hang from the ceiling of a cavern are known as stalactites. These cave formations are composed primarily of calcium carbonate. The formation process starts when rainwater, which has absorbed CO2 from the atmosphere and becomes slightly acidic, seeps through limestone, dissolving calcium carbonate along its way. As the water drips through cave ceilings, it leaves behind deposits of calcium carbonate, which over time grow into icicle-shaped structures hanging downwards, referred to as stalactites. Below them, on the cave floor, dripping water may also create upward-pointing structures called stalagmites. Other related formations include columns and dripstones, which can also be found in caves.