Dig a 500 foot tunnel by hand?

You need to do some serious research first. Read up on tunnel shoring and ventilation, as those will be your biggest hazards (forget the liquid oxygen, that's way too expensive for the length of time you'll be in there). Moving the dirt will be your next problem. Dirt weighs about 90 pounds a cubic foot, and a 500' tunnel (uming a 4'x6' cross section) will yield 12,000 cubic feet, or over 500 tons of dirt. If you solve all those problems, the actual digging will be relatively easy (picks and shovels should do the trick--unless you hit rock).