system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why and how do caves form inside mountains or rocks?

I understand somewhat that the formations inside these caves are caused by water/creeks and the shapes inside are from droplets where crystals form over thousands of years. But how do some become massive and so open?
gpt
Either running water or magma.

A tiny crack in the rock where water runs down can turn into a massive cave over the course of thousands of years. In volcanic regions hot magma carves caves and tunnels out of the rock.