when i was little, we went to lanzarote, and there were caves there, we had to go down a long way and they said they had something amazing to show us, when we got there they had a big pool of water like a small lake, the toor guide said, keep all the children back from the side, its miles and miles deep,reflecting all and also extreamly cold...
I went to Cueva de los Verdes, too... what a neat place! The tour takes you all around the cave, including to its underground concert hall (they have a few performances each year, I think).
You have remind me having the same wonderful experience myself. I also was little when I went there. Even more, I can never forget what happenned to us: The guide chose my mother to throw the rock in the huge cave. She was carrying a bag with all the photographic stuff and it went ahead from her with the movement of throwing the rock. She felt she was falling into the enormous hole. She almost had a heart attack!