You'd have to put them on top, after 100 years in the ground you'd still be quite intact.  Its quite common over here to be planted above your other-half, thats why they go 6ft.  In ye-olde worlde the original use of a brass plate on the coffin lid was to identify how deep they were.  Grave diggers would have a reasonably good idea of where the plate would be and would use an iron rod to drive through the ground to hit the plate, thus finding how close they were getting!