The Grotto of the Redemption in West Bend Iowa is amazing. It's the largest man made grotto in the world and the work of just a few individuals starting with Father Paul Dobberstein. You don't have to be Catholic (or even religious) to appreciate it.
The stones that make up the Grotto have been valued at over $4,000,000
The kids had no idea what this was, but we all fit in it.