Cave popcorn, or cave coral, is a very common cave formation. It actually comes in many sizes, shapes, and colors, but is most often found in small, knobby clusters resembling popcorn. It is considered a subcategory of a more diverse type of speleothem called a coralloid.
Cave popcorn is composed of calcium carbonate and forms by several different mechanisms, including deposition by seeping water and underwater deposition on the walls and floors of pools. Sometimes, cave coral forms as knobs or branching stems that extend several inches from walls or floors.