its not terribly old, built in the 70s (in eastern canada fwiw). the house is on a septic system that we know not to be located anywhere near this cold room as well if that means anything.
It has also crossed my mind that it may be possible this "dirt" is literally all just well rotted wood. The floor is sitting on completed rotted out legs, many of the undersides of the board are rotted out and there was a piece of board looking mid-disintegration on top of the pile that appears to just be becoming one with the dirt.
The main reason I dont think that can be the whole story is just because of the volume of it.
Thanks