Although the phrase "six feet under" traces back to a centuries-old English law, modern graves in the United States are as shallow as 4 feet. In reaction to the 1665 plague outbreak, the mayor of London enacted a law requiring that all graves be at least 4 feet deep.
As of 2012, most states require at least 18 inches of soil above a casket, or 2 feet of soil above an unenclosed body. In New York City, if the casket is contained within a concrete vault, the top of the casket must have at least 2 feet of soil. If the casket is not in a concrete vault, at least 3 feet of soil is required, although there is no state requirement. With the average casket being about 30 inches tall, a casket buried without a concrete vault would require a grave 5 feet, 6 inches deep, which is nearly the 6-foot depth of the dated law.
The typical modern burial includes an embalmed body sealed in a metal casket; however, there are alternatives. A "green" burial, also known as a "natural" burial, involves an unembalmed body buried in cloth shrouds or a simple coffin of cardboard or soft wood, such as pine. According to Grave Matters, this practice is a return to a once-standard practice in the United States. In this state, the body rejoins the natural elements over time.