(from http://en.wikipedia.org/wiki/Stalagmite) are 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. This stalagmite formation occurs only under certain pH conditions within the underground cavern. 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.
As all flowstone formation, this is a secondary stone and at times quite fragile.
The speed of growth is depending on several factors but in general you can say that stalagmites grow slower then their counterparts. Once submerged in the water, these formations do not grow any more and also don’t erode thus are presenting a window into time.