Conversation so far:
how far does a coffin go into the ground
Cemetery graves in the US are not always 6 feet deep, and for single gravesites, roughly four feet deep is closer to the norm.
where does the six feet under standard come from
It all started during the 1655 bubonic plague. The mayor of London immediately issued a law on how to properly deal with dead bodies to prevent the spread of infection.

Question: why is a vault necessary
Self-contained rewrite: why is a burial vault neccessary