I hope you know where any and all utility lines are running around on your property, because you might think, "well the power line is right there, so the line must go underground here!"
is not always the case, and you say this is in a shed correct, what kind of bracing and shoring are you going to be doing on the overhead and sidewalls....which honestly if you arent doing a poured concrete floor with rebar, nothing will hold the sides in short of digging holes and pouring concrete and rebar for support beams in the corners, I say these things because digging and building underground, ESPECIALLY COVERTLY with no real knowledge of structural engineering is pretty foolish to say the least.
Not trying to rain on your parade BUT building is kinda my blood....and I have done underground construction....its not easy. ANY ?'s though and I would be happy to answer them.
I just saw the rough sketches you got a little clearer 4' to surface, better have 5k minimum in rebar and concrete and quite a bit of 4x4's and 2x6's and 2x8's and FUCKTON of 1" PLYWOOD not barkchips glued in a pancake fashion pressboard crap....real PLYWOOD 1" THICK DOUBLE LAYERED or you will be buried alive like a trenchdog in nam.
No offense, but there's no need for such intense concern, I'll be ok. Admittedly, there is a lot more to this than I thought at first. However, that only means that it will take longer to do. At first, I had some haired-brained idea that it would only take about a month... Haha, joke's on me. As far as final plans go, I have no idea what I'll wind up constructing until I'm through. I Know it sounds bat-shit nuts, but "the room" will "tell" me what I need to do. It's ok, I've built things with that concept before. Believe it or not, it works really well. Kinda like functional art. I do have a few concrete facts though:
1. I won't be hitting any power lines. Those are VERY rare here, and I'm already under a pretty solid layer of rock. I can tell by just looking that no human hands have worked this earth during any time in the near geolgical past.
2. I'll be using R/R ties for support, much like in traditional mining. As far as ceiling material, I'm not sure. It needs to sturdy, water/rot-proof, and "somewhat" easy to handle. Remember: I can support this from underneath.
3. Finally, I am making sure that the area directly above the room will NEVER have to support a great deal of weight. That way, I won't have to make it absolutely "bombproof".
I admit that I don't have much experience with construction, and I will have many questions. Safety, however, won't be much of an issue, since I over examine everything. It would help to point out more subtle hazards than the obvious, however. The faraday cage, for instance. Gonna have to look at that some more...