Buoyancy.....during the floods of "93" alot of coffins popped out of the ground because the ground was so saturated. Lock down the lid and the remains stay in place.

Cruel Joke:
Make a small coffin and put your victims name at the head end. Make sure the center of gravity is closest to the head...may have to add a bit of extra wood to the foot end. Bury it about 3 ft. deep in a garden so the new hole won't be noticed. The first time ya get a heavy rain so the ground saturates that deep the coffin will pop out of the ground head first like on Poltergeist.

LOL....I pulled that trick on a friend one time, freaked the shit out of him!
I let him sweat it out for a couple days then had to tell the dude it was me.

Have a good one!