The term 'six feet under' refers to a grave.
The concept of a burial being six feet deep goes back to a time in our history when people were not buried in concrete, metal or fiberglass outer containers and sometimes not even in caskets. This depth was thought deep enough to prevent animals from digging up the grave.
Often a grave will be six feet deep at the bottom. Once the casket and outer burial container are in place, this will usually leave
about three feet of dirt on top.