The common phrase "six feet under" is traced back to England in 1665, when an outbreak of the plague led the mayor of London to enact a law requiring all graves to be at least six feet deep in an attempt to limit the spread of disease. Today, many bodies are not actually buried six feet underground, but laws still exist to mandate burial requirements.
If bodies are not buried deep enough, a few years' worth of soil erosion could expose them. Not only is this unsightly, but there is a common fear that dead bodies spread disease. In reality, very few diseases can be contracted from dead bodies.
Many graves today are only about four feet. Burial laws vary, but 18 inches of soil on top of a casket is a common requirement. If the body is not enclosed, two feet of soil is often mandated. In some places, such as low-lying wet land areas, graves must be much deeper than six feet, otherwise they would fill with water.
It is also important to bury coffins well underground to protect them from grave robbers and body snatchers. In the 1800s, body snatching became a lucrative business, when human cadavers could be sold for research.