Fairly low,at least a couple of feet below existing ground surface. But remember archies dig graves for science, so whatever they do is OK.
Practices and standards in dealing with human remains have changed radically during my time, and today, much more consideration is given to the wishes of the descendants of those we are digging up, which sometimes means the graves are not disturbed at all or are reinterred by native groups after comprehensive study.
There is a real difference between North America, where recent immigrants are studying the original inhabitants, and Great Britain, where the natives are digging up the natives. I really liked the way the remains of Richard III were handled, appropriate reburial after thorough study.
Indiana Jones has been both a curse and a blessing. People now know more or less what an archie does. That wasn't the case, pre IJ....
And the shovels we use are just plain old hardware store issue.
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Geezer in Chief