I've been working on a medium sized underground (18 feet-ish deep) storage area. It's a little odd though, because it requires a person be capable of fitting through a small enough hole to gain entry. Anyone who is overweight will have a tough time. Still not sure what final size will be for the door, probably better to push it on the smaller side for security reasons.
Considering the option of adding some 4" thick steel plates on the ceiling, fastened with roofing nails of course, to help prevent entry from above. Will take a long time to cut through that.
There is one problem I've been dealing with, and that is water intake from the house plumbing above the area. Somehow it keeps leaking what I believe is toilet water into the lower space. Smells terrible. Current solution has been to keep expanding the area and let it drain off into the lower portions. It's starting to overtake the rate that I can dig to expand, so I'm open to ideas on how to fix this problem.
Hoping to start running three phase power down to the area so I can stop using gas lanterns to light the space. It's a bit intimidating as I have never run electric wiring before. Shouldn't be too bad though, the guy at Home Depot gave me some pointers.
As for where valuables will be stored, I'm thinking of re-purposing a few refrigerators to act as lockers. Once I get the power hooked up, I'm hoping to drag a few down there and use them to control the climate of storage inside. At least half will be left open all the time to help cool down the space as I don't have a reasonable way to get AC functioning.
One cool thing that wasn't planned and just sorta happened, I discovered a natural gas line while digging outwards. I plan on tapping into it for a small NG generator hookup, just in the case my main power line I run gets cut or whatever. Probably going with a 5kw-7kw generator unit that I will stick in the corner. Should power all my fridges to help keep the place cooled down in the hot summer months.
Thinking out loud while typing this, I don't see why I couldn't also hook up an NG heater as well. I'm open to tips on how that could be setup if anyone else has experience.