A big part of the labor involves moving the sand and clay from under the house to the backyard and if you've ever watched Escape from Stollagg 17 and really thought about the dirt in the socks trick, a Yard of Sand is a Lotta Socks...
I started with a crawl space under about half the main floor of the house, and about 4 foot of head room, and a 3 by 4 foot doorway halfway up the steps or four feet above the bottom garage floor.
Pyramids have always fascinated me, the shear tonnage of them and I always wonder what they are resting on, the backgrounds always seem to be sand that can flow over time and they have stood for so long a simple shape transferring energy over a large base and when I was thinking of the bunker Idea realizing in Florida at some point I was going to dig into a water table and a heavy pier might over time sink like the Kansai Airport, KIX not being built by kids and needing hydraulics and such to keep it sitting level I thought that an inverted pyramid would be less likely to sink since to go down you'd need to also push some vector off in a horizontal type way and then I thought some more and having been hit in the head but not necessarily made foolish I guestimated 2 feet as about the maximum cliff you could dig into a seam of sand and have it expect to stay put as long as the sand was kept damp and if it collapsed it would probably go to a 45 or so meaning if I stayed more than two foot plus a fudge factor away from anything that might be adversely effected by having it's legs pulled out from under it, in the most broad sense of the term it should be fairly safe...
I added 2 and 2 and a few more and decided the simplest thing to do would be to dig down two feet, form a temporary wall about 4 inches thick, drive some rebar a few feet into the ground and tie them to some running horizontally inside the pour, carry in a few hundred 80 pound sacks of concrete mix, pour the wall, telescope in and repeat the process a few time digging down to pull the rebar from the first pour at and angle and tying it into the rebar for the next one and then when it was 4 or 6 or however many feet high depending on the load next to it I'd form one more wall and end up with vertical smooth walls that are thicker on top than they are at the bottom just like an upside down pyramid.
The deeper I go the less floor space I have but it seemed doable and since I watched the well be put in and dug the very large hole seven years ago into the hill so I could have a level mostly below ground for hurricanes and such, this being Florida after all and things happen, I had an idea of what type of soil was down there, and new there was a seam of clay that was fairly thick and guess that since the lake is about ten feet below the height of the bottom garage I probably could come close to being able to dig down at least one story without it turning into a sinkhole but I also new it would be unwise to just start building something without first taking a look at what was really down there so I decided on a test shaft to test construction methods and get soil samples at certain depths and most importantly how deep you can dig without hitting water since dampness and closed up places don't go well together and an ideal emergency shelter is ready at a moments notice even if it has not been maintained for a decade.
At least I think so since I have seen many 1950's era bunkers built for the cold war that are now long flooded because it costs money to run a sump pump.
Fort Walton Beach is full of them if you look hard...
Step 1: Walk Like an Egyption
This was just sand but under normal situations the crawl space stays very dry which is more dumb luck than anything so I would need to spray it with the hose a bit to soften it up since there is a small amount of clay in it as well.
I started by cutting a sheet of plywood wide enough to fit over the steps forming a ramp between to block walls filled with concrete once again having spent many moments pondering the building of Pyramids it's entirely possible I meant to do this when I first laid the blocks for the bottom level just like how the supporting columns in there holding a beam so I can have 20 foot spans above are spaced so you could knock a hole in the back wall of the bottom garage in an area that just happens to not have rebar like it was meant to be sawn open with a partner saw and then drive a BobCat under there and go to town but that would imply long term planning and possibly lead to a much longer post than intended but it was pretty simple to shovel sand out the opening then when I got a big pile shovel it into wheel barrows and use it to fill in lowspots in the yard.
And it was good exercise!