Stalactites form when drips of water fall from the ceiling of a cave. Before dripping drops deposit a tiny amount of calcite (calcium carbonate). Over time this builds up on the ceiling and a tapered growth extends downwards. In places where there are many drips, such as this cave at Waitomo, a whole field of stalactites can build up. Stalactites can extend so far downwards that they meet stalagmites (which grow upwards from cave floors) forming columns.