'We' have the Great Plague of 1665 in London to thank for the expression "6 feet under." This tragic recurrence of the Bubonic Plague (which first decimated Europe in the 14th century) was the impetus for the law requiring dead bodies to be buried at least 6 feet below ground. The comprehensive cemetery site, City of the Silent, addresses the matter on its page of frequently asked questions. English law once required a burial depth of 6 feet to ensure the corpse didn't spread the plague to the living. Of course, this measure was ineffective as fleas infected with the plague probably spread the disease. Also, few diseases are contracted from contact with dead bodies. The same site gives a rather gruesome description of early cemeteries as being "littered with bones and bits of charnel." Furthermore, shallow graves allowed scavengers (presumably both human and animal) to easily dig up the remains -- which makes a 6-foot-deep burial seem like a decent idea. Furthermore, it's a tradition to speak of 'six feet under, but it is no longer the law, with many places in the U.S. requiring lesser amounts. http://www.alsirat.com/silence/

In Australia, the minimum is also 6 feet down, however most grave sites counter for at least two bodies to be buried in the same spot at different times. I think the first coffin goes in at 9 foot and when the other partner dies they re-open the hole to about 7 feet down, and place them on top, so really, they are six foot down.

Except for doubleburials, few graves in the US are dug deeper than four-and-a-half feet; many are stopped at four feet. The use of sturdy containers prevents grave-robbing by people or varmints, so the extra depth is seen today as unnecessary. That said, it's interesting that the warranties issued by some manufacturers of metal vaults (used to enclose a casket in the grave)require the base of the vault to be at least five feet underground or the warranty is void.

The ROT about the 6' depth was that no animal was going to dig that deep to get to a body.