Since there was interest I am writing this although it isn't for all situations but I will cover many things to think about and others can add more info.
First I will address things brought up from posts concerning this topic.
Locating the room/cache.
As stated this is very important but not too hard to do. In determining possible under slab water pipes you find the shut off valve on the exterior of the building which is where the water enters the house. Older homes most often have water pipes and elec. wiring within the walls but regardless you need to be sure. If under the slab the pipes would be in the soil 12''-16" and would usually be in line from the exterior shut off valve to a kitchen, bathroom, water heater, clothes washer- any water source. So you would avoid areas in line with water points.
Gas lines run from the gas meter to the kitchen, water heater, pool heater, chimney and heater/furnance but are usually run from point to point in the walls.
Electricity enters the house from your electrical panel and is usually in the walls but if under the slab goes to major points of use like a kitchen and from there through the walls.
Waste pipes run out to the street to drain into the sewer. Sometimes you will see a clean out opening cover just outside the house indicating the waste pipe below exiting the house to the street. A main 3" waste pipe will run from the furthest toilet to the street, with small lines connecting sinks and tubs to it. Waste pipes will be the deepest down 3'-7' down. Naturally each toilet, sink, tub-all drains will connect to the main waste line at some point. These would be the most randomly laid down being up to the plumber as to where to lay them. But due to their depth are in no danger of breaking.
The above info is to inform the factors to consider but the natural areas, in my mind would be in bedroom closets in those closets away from utility "centers". In your typical clothes closet or a walk-in closet. Having a carpet to cover the "door/opening" after completion is needed to hide it otherwise a garage or a storage room would work.
Now to business. Having found your spot pull up the carpet off the tack strips (easily put back when done). If it is beside an exterior wall start at least 6"-"8" from the wall to avoid the larger concrete footing below around the perimeter of the house. Larger bearing footings can be found toward the centerline of a building to support the structures above and again coming a little away from the wall would avoid the footing below. On the bare concrete pencil off an opening using a square to make all corners 90 degree corners, 2' x 3' sounds like a good size, big enough to get into and materials into, unless it is strictly a gun cache then 2' x 2' would work. You need to "score" the concrete by using a masonry blade on a circular saw and cut down 1/2" inch to make a clean hole opening, cut past the corners so the concrete will break at the score. This is a dusty part so take 1 mil. (thickness) painters plastic and surround, "bag" the work area to contain the dust. Wear glasses for eye protection, a filter mask, and earplugs. It would also be good to wrap in plastic the vent openings of your saw motor to keep out the dust.
After getting to this point a 60lb. electric jackhammer with a point bit can be rented for half a day for about $25-$40. Starting in the middle Jackhammer through the 4" of concrete. Make a number of holes to make a small opening and then cut around this opening enlarging it. Finally break out carefully around the score cut for a clean square opening. Get rid of the concrete and from there on you have dirt. By the way this is what you would do to install a floor safe. Using 2 - 5 gal. paint buckets fill with dirt 3/4 full or more to remove dirt from the hole. A wheel barrow would be handy to move the dirt out to the yard to dispose. Initially just dig a 2'x2'x3'deep and go on from there. Cutting 3' widths of the painters plastic and taping them on the floor with 2" painters tape will keep your house /carpet/floor clean while removing the dirt to the back yard . Covering the hole temporarily with some plywood would work. The work to this point. is about 7-8 hrs. worth. Without interference below, a room as large as 7'x7'sq. x 6ft deep could be made with only the slab above without harm. You need to avoid undermining the supporting footings although a 3'-4' tunnel under a footing would do no harm. Also if you were near the perimeter of the house an escape tunnel could be made. If you wanted you could frame a floor and walls in the room and even cover with plywood or drywall. For the hole cover you could use 3/4" plywood setting on metal or wood cleats attached to the hole walls making the plywood top flush with the top of the concrete floor. Or for a more solid and undetectable hole cover you make a concrete cover that sits on metal cleats/shelves that flushes out even with the existing concrete floor level. Buy 2- 60lb. sacks of concrete mix, a length of concrete wire mesh (to place inside the concrete of the cover to reinforce it), a 10' piece of 2"x4" wood, and 2 3/8"x8" bolts, nuts and washers to use as a handle on the cover (all for about $12). Make a frame slightly smaller than the hole opening and make sure it is square. Cut the mesh to fit inside the frame, mix the concrete and pour in 3 1/2"-4" thick (try to keep the mesh in the middle) and finish with a $2 trowel. While the concrete is wet take the 2 bolts and push into the concrete sticking straight up) in the centerline of the cover 6" from the ends to be used for handles to lift the lid.
The last thing to do is to make a shelf/lip for the cover to set on, and be flush with the top surface of the floor so that the cover cannot be felt through the carpet over it. For this you could use 2 -1/4"x4" (or more) x3' steel plate with 2 hole towards the ends of each piece. You you also need 4- 3/8"x6" bolts, nuts and washers. Line up the steel plate down the 3' lengths with 1 1/4" of the plate hanging into the hole opening to act as a shelf/lip down each side of the opening , mark the holes and drill through the slab to place the bolts through and bolt the plates solid to the bottom of the slab. Take the new cover when it is hard and make sure it fits into the hole and sits on the shelf. (With the shelf bolted from below you could unbolt the shelf and remove the cover from inside.) It is probably not flush/even at the top all around but you can mix some mortar and glue, spread it across each shelf , let set up a little and place the lid into the hole (setting on the shelf) pressing down until it flushes out and leave it until the mortar sets up. Now it will be flush and solid when someone walks on it. Put the carpet across it and you wouldn't know it was there.
I think we are coming to the point in time that something like this would be nice or even necessary.
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