A cave is a natural opening in the ground with a hollow space beneath the surface large enough for people to enter.
This acid allows it slowly dissolve certain kinds of rock like limestone, marble or gypsum.
As the water drips and later evaporates, it leaves behind a thin layer of minerals.
Rain has a tiny bit of acid in it and when it soaks through soil, it collects some more.
While stalagmites come up from the floor of a cave like a column.
These formations are created when water carrying minerals trickles through the cave.