Emptiness inside hills, mountains and rocks are called caves. Their origin is different. Sometimes they arise from the fact that the waves beating against the rocks, "gouging" them in empty space. And there are caves beneath the water. They appear in the groundwater, which wash soft rocks such as limestone. You already know that the earth's crust moving. The mountains are also in constant motion. In the progress of rocks also occur caves.
Often there are caves with holes in the ceiling. These have holes where the accumulated water, which then broke into the cave.