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.