“This means that whatever was painted on the outside, supposedly long ago, should be covered up now by more CaCO3 deposited in the interim. No?”
Maybe. Depends when the stalagmites stopped “growing” Eventually, they reach a size where the water can’t flow down them, or comes down unevenly.
Aside from the “art” issue, being this deep in a cave shows NT (or whomever) mastered not only fire, but mobile fire used for lighting a cave. That’s huge.
Do you think Adam and Eve had to “master” carrying a torch?