George Lucas wrote:

I don't know if you can rent one of these or not, but the easiest way
to do it would be a Portable Electric Concrete Core Drill:
http://www.durhamgeo.com/testing/con...nd-coring.html
(third one down). I used one a lot when I worked for a materials
testing company and had to take asphalt cores to check for road
thickness and density. Hook up some kind of hand-pumpable water
supply tank, plug it in and it would make short work of your problem.
It looks like this model can drill holls up to 10" in diameter. Just
make sure to keep pumping so it doesn't run out of water and once the
water turns brown you've hit dirt.We had special tongs designed to
remove the core once it's cut, but I'm sure you could figure
something out to grab it and pull it out.

You can rent them.

After your postholes, drill a hole in a closet for installing a below-floor
safe.