A stalactite is a cone-shaped formation of rock that hangs down from the ceiling of a cave, like an icicle made of stone. Stalactites form slowly over thousands of years as water containing dissolved minerals drips from the cave roof. Each drop leaves behind a tiny bit of mineral (usually calcite from limestone), and gradually these deposits build up into a hanging spike that can grow many feet long.

You can remember which cave formation is which by thinking that stalactites hold tight to the ceiling, while their counterparts, stalagmites, grow up from the ground (they might reach the ceiling someday). Sometimes a stalactite and stalagmite will meet in the middle, forming a complete column from floor to ceiling.

These formations grow incredibly slowly, often just a fraction of an inch per century. The giant stalactites you might see in famous caves like Carlsbad Caverns took tens of thousands of years to form. Breaking one off would destroy something that took longer to create than many human civilizations have existed. That is why cave explorers and scientists treat these formations with such respect, avoiding touching them (the oils from human hands can actually stop their growth).