These are formations found in limestone caves. Stalactites are columns of limestone that hang from cave roofs, whereas stalagmites are columns of limestone that grow from the floor of the cave.

In each case the source of the limestone is the continual dripping of lime-rich water from the cave roof. As the droplets enter the cave, carbon dioxide escapes and this causes the deposition of calcium carbonate (limestone). Stalactites are more slender than stalagmites and grow at about 2mm/0.1in a year on average.