Why do we bury our dead six feet under?

You may be familiar with (6 feet underground) quote. It is directly connected with death but did you ever think why the dead people bury six feet under the ground.

In 1655 years, England was affected by Bubonic plague and the governor of London ordered to bury the dead people six feet under the ground to stop the spreading of the epidemic.

However, they found later that there is not a connection between the burial process and the epidemic. Therefore, 6 feet low was not in used anymore.

Finally, it seems there is not a standard for burial depth.