I’ve always found caves comfortable, and I especially liked Cave of the Mounds. After a day of blistering heat that drove us to our motel room for the bulk of it,the cool underground tour came as a relief.
I learned in the process of the tour that the official definition of cave is “large enough to fit a human.” The size of human – and thus the size of hole – has not been standardized. I also found that the cave had a magical quality to it, like I could talk to gnomes and feel quite comfortable down there. Even the moments of pitch black didn’t bother me at all.