The reason we bury the dead and dead people down 6 feet in the ground is to prevent spread of possible disease.
It used to be that a body could be buried at any depth but the law changed that says that we must dig a grave to at least 6 feet to bury the body.
You can dig deeper and bury the body deeper if you want too and that happens sometimes as in a spouse dies and then they dig and bury the body 12 feet deep.
Then they later bury the other spouse on top of the other casket at 6 feet.
But for one body it must be buried 6 feet minimum down to prevent the spread of diseases.
The issue came up about the plague disease many years ago when people died of the plague.