Talking of caves which are underground, I've also been down a few mines.
One notable one was the Wielicczka Salt mine in Poland.
The miners carved great caverns from the rock salt, often with carved sculptures of various figures surrounding them. This started hundreds of years ago,
in modern times electric lights have been added and the mines turned into tourist attractions, as you can see in the above picture.
Read about this salt mine:
https://en.wikipedia.org/wiki/Wieliczka_Salt_Mine
Anyone else been down a mine?