Stalagmite

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Stalagmites may also refer to a type of fungus

The Witch's Finger in the Carlsbad Caverns

A stalagmite (from the Greek stalagma ("Σταλαγμίτης"), "drop" or "drip") is a type of speleothem that rises from the floor of a limestone cave due to the dripping of mineralized solutions and the deposition of calcium carbonate.

The Hall of Giants in the Carlsbad Caverns

The corresponding formation on the ceiling of a cave is known as a stalactite. If these formations grow together, the result is known as a column.

When touring caves with stalactites and stalagmites you might be requested to not touch the rock formations. This is generally because the formation is considered to still be growing and forming. Since the rock buildup is formed by minerals solidifying out of the water solution onto the old surface, skin oils can disturb where the mineral water will cling, thus affecting the growth of the formation. Oils and dirt from the hands can also stain the formation and change its colour permanently.

Stalagmites can also form in lava tubes, although the mechanism of formation is much different. Stalactites and stalagmites can also form on concrete ceilings and floors, although they form much more rapidly there than in the natural cave environment.