There was an old trick played by the grave diggers in order to save themselves a bit of effort when they knew that they would be digging down six feet through hard soil only a year or two later to reunite husband and wife in the afterlife. Instead of filling the grave with earth after the ceremony, they put wooden planks about 6 inches under the turf, then covered them in soil for the mason to place the stone on. This was more common in 'table' tombs which stood three or four feet above ground and had 4 walls and a roof, but - in this case - they did it with a flat one.
After a few years, the boards rot and can no longer hold the enormous weight, so the stone goes down to join the occupants of the grave.
When I was a kid, me and a friend went into a graveyard and found a table-tomb which had one panel missing from the side, and my mate decided he was going to crawl in to investigate. He went in and laid down flat. In one second he fell 6 feet through the thin layer of soil and rotten wood, then fell through the rotten lid of top coffin beneath, finding himself lying amongst the bones of the last person to be interred there, about 150 years before.
I have never seen someone get out of a deep hole so quickly.