It's a large hole, in a rock. Even as a geology nut, I did not find it that impressive. What was impressive were the hundreds of others who were treating this mediocre rock hole like it was a day at the beach. I was surprised to not see these people sunbathing on beach towels. Sheesh. 5 minutes and I was out of there.
I would like to say it's worth doing once, but I'm seriously not sure about that.