Ah but Mark, you're making the disasterous mistake of using common sense logic. I mean, ... c'mon, ... we all know those person who buried that "certain treasure" there 200 yrs. ago, got all sorts of pumps, wall-barriers constructed, and so forth, ... to put that treasure way below the water table line of that terrain. But alas, your common sense questions will have no effect on this. Any attempt to explain away the treasure lead (as embellished superstition), or common-sense reasons why it simply can't have been buried at such depths, will fall on deaf ears.
I'm still trying to understand, how 1 ft. vs 10 ft,, makes any difference at all in the "hiding it" process. I mean, sure, now in the last 40-ish years that detectors have become widespread, I suppose someone can make it deeper to foil those with metal detectors. But prior to the advent of detectors, what the h*ck difference did 1 ft, versus 3 ft,, versus 5 ft, etc... make? As long as the surface was covered back over, then going deeper doesn't hide it any better. In each way, it's still invisible.
About the only plausible explanation anyone has offered to this question, is if jungle terrain tree-shedding, (or mudslides or something) covered it deeper. But for the initial person to bury it that deep in the beginning? Notice that in the OP's case here, he's saying it's in the floor of a concrete building. Hence not jungle terrain, mudslides, etc...