Here's how it happens:

* Water seeps through the ground: Rainwater, slightly acidic from dissolved carbon dioxide, seeps through the ground and dissolves the limestone.

* Minerals are carried: This water carries dissolved calcium carbonate (CaCO3) and other minerals into the cave.

* Evaporation and deposition: As the water drips from the ceiling of the cave, it evaporates, leaving behind the dissolved minerals. These minerals accumulate over time, forming stalagmites on the cave floor.

So, stalagmites are specifically formed in caves made of limestone, which is a sedimentary rock primarily composed of calcium carbonate.