I've long had a fascination with caves. When I was a boy we had a sandstone cave of sorts in our backyard (we lived on 5 acres of undeveloped land in the hills north of Sydney) and I would build fires and imagine I was an aboriginal hunter. Anyway, my Dad is intensely claustrophobic and I've always enjoyed giving him the willies by describing my adventures in various cave-like places such as attics, sub-floors, and crawlspaces. A few years ago, I was on vacation with my college buddies (we all ended up back in the Bay Area after school) in the California gold country, and we came to a place where they had a guided cave tour. As in - real honest to God crawling through muck caves. About six of us went in for it (the others went hossback riding or something), hooked up with 3 more people and a guide, and spent about 4 hours crawling around in the belly of the Sierra Nevadas. My favourite part (and the one my Dad leaves the room for when I retell the story) is a little side chamber which has fairly stringent entrance requirements. Its about 200 feet below the surface, and you need to put your arms up, push them into a hole in the wall, exhale and slide your body through the hole about 2 feet into the chamber. The rock has been polished smooth after decades of people doing this, so its physically not difficult (provided you're of moderate proportions. Once inside, the chamber is about 4 feet in height and eight feet in diameter. The best part is once you're in, everyone turns off their light. That, my friends, is darkness. There is not a spare photon to be had.