My rest day was great -- and just what I needed. The caves were a welcome alternative to riding through hills -- and at a steady 54 degrees, the temperature was a welcome break as well.
I ran into the Dutch again, though they were only visiting for a few hours. They were camping about 30 miles north of the park, and had ridden in without their equipment.
Here are a few pictures of the caves. Sorry I don't have more, but the park service forbids the use of tripods in the caves, so I couldn't take low-light pictures.
The last photo is obviously backlit. It comes from a portion of the cave called "Frozen Niagra," which -- unlike the rest of the 365-mile cave system -- contains stalagmites and stalactites. The rest of the cave is covered by a sandstone ceiling, which prevents water from seeping down into the cave.