What Am I?

Clue 1: I am a hole used by animals but I am not a burrow.

Clue 2: You will find me underground or in the side of a cliff or mountain.

Clue 3: You will sometimes find lakes and rivers deep inside me.

Clue 4: Stalactites hang from my ceiling and stalagmites grow up from my floor.

Clue 5: Animals like bears and bats take shelter in me.

Clue 6: Prehistoric people used to live in me and draw pictures on my walls.