A stalactite (Greek stalaktites, from "drip" and meaning "that which drips") is a type of speleothem (secondary mineral) that hangs from the ceiling or wall of limestone caves. It is sometimes referred to as dripstone.
Stalactites are formed by the deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions. Limestone is calcium carbonate rock which is dissolved by water that contains carbon dioxide forming a calcium hydrogencarbonate solution.
This solution travels through the rock until it reaches an edge and if this is on the roof of a cave it will drip down. When the solution comes into contact with air the chemical reaction that created it is reversed and particles of calcium carbonate are deposited.
An average growth rate is 0.13 mm (0.005 inches) a year. The quickest growing stalactites are those formed by fast flowing water rich in calcium carbonate and carbon dioxide, these can grow at 3 mm (0.12 inches) per year.
Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (0.5 mm), hollow tube commonly known as a "soda straw" stalactite. Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite. The same water drops that fall from the tip of a stalactite deposit more calcite on the floor below, eventually resulting in a rounded or cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws." Given enough time, these formations can meet and fuse to create columns.
Stalactites can also form in lava tubes, although the mechanism of formation is much different.
While it has been claimed that the longest stalactite known hangs in the Chamber of Rarities in the Gruta Rei do Mato (Sete Lagoas, Minas Gerais, Brazil) and is 20 metres long, vertical cavers have often encountered longer stalactites while exploring. One of the longest stalactites viewable by the general public is in Doolin Cave, County Clare, Ireland, in a karst region known as The Burren, what makes it more impressive is the fact that the stalactite is held on by a section of calcite less than 0.3 square meters. The White Chamber in the Jeita Grotto's upper cavern in Lebanon holds an 8.2 meters stalactite which is also accessible to visitors and is claimed to be the longest stalactite in the world.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)