Cause anything under 6, you can still smell them.

I guessing its to keep them below the frost line so that repeated cycles of freezing and thawing doesn't pop them up out of the ground?

When I buried my mother the top of the coffin was only 18-inches or maybe as much as 24-inches. When my uncle was cremated he was buried on top of his wife's coffin. The were unable to go down more that 18-inches before they hit her coffin. The head is no more than 12-inches from the top of the coffin. So they aren't buried 6-feet down. The hole does go down 6-feet though.

Six over takes up too much space and is a bit morbid.

I heard that that is the depth it needs to be to keep the smell from rising out of the ground...Keeps animals and such from smelling it and digging it up.

laws and regulations vary from area to area, even within UK there are differences. The most common regulations state that no part of the coffin may be less than 0.9m (3 feet) below the surface of the ground. Many people have a "family plot" - either a single or a double plot. A single plot allows for 3 coffins to be buried on top of one another, whereas a double allows for 6 in total - 2 piles of 3 deep. A gravedigger I once spoke to told me that these are buried at depths of 8, 6 and 4 feet - allowing for the average coffin to be a foot high that means that the top grave will be 3 feet under the ground. In England, we currently face a space issue when it comes to graveyards, and so discussions are afoot in many areas to re-use graves. This is normally done 100 years after the burial - by which stage, only skeletal remains are left. The gravemarkers (headstones) are usually moved to another area of the graveyard - usually, round the edge, and then the plots are reused for another 3 graves. However, the current trend of cremations is making that a lot easier, as ashes take up a lot less space than a full burial plot. Many graveyards now have areas set aside for cremated remains. My local graveyard is reaching the stage of being full, and so it will be come necessary to either re-use graves, or to send people away! There are, however, 'historic graveyards' which are exempt from being re-use - examples of this are Highgate cemetary in london - where good old Karl Marx is buried. So.. 6 feet under? for us, it is only an expression.