Are there any scanners I could hire/buy (if cheap enough) to scan the floor and make out the shape of the cellar below?.
Old, but interesting thread, none the less...
GPR, is what might help. Ground Penetrating Radar. The things Timeteam sometimes use, to reveal long buried building foundations and etc.. They drag a box along the ground/floor, it then records what is directly below, as a sort of depth display, but the display needs some interpretation.