I was trying to track down an origin for the popular claim (Tom Cox, Bill Bryson &c) that many English parish churchyards have circa 10,000 bodies buried in them (with an often stated 40,000+ in Haworth), and the parallel claim that the reason visitors step *down* to floor level in many old English country churches (and the walls often visibly continue below current ground surface) is because so many bodies have been interred in the graveyard outside (with soil intermittently added above for obvious reasons). Note: land surfaces in densely populated towns and cities tend to rise more rapidly than in rural areas (except e.g. Venice...) so there isn't any one explanation for every church.
Some interesting and relevant quotes from popular articles at present easily available online (no academic references, although the quotees are *cough* respected in their fields [/pun]). WARNING for discussion of human remains and if you click through then occasional images of bones (extra warning for clicking through and reading the final link, which also has a separate warning).
( Quotes (warning above) )