Once upon a time in the Middle Ages (in 1278), a Cistercian abbot in what is now the Czech Republic brought home some earth from the Holy Land, and he scattered it on the local cemetery. This gave it the status of very holy ground, and made it an especially desired place to leave one's mortal remains. In the next forty years, over 30,000 of the faithful were buried on that sacred site.
In 1511, bones from the graves were relocated into a chapel crypt so that the graveyard could be re-used (I'm told this is not unusual in Europe), and in 1870 an artist was given the assignment of turning the bones into decorations for the interior of the chapel. The results of his devotional artwork are there today in the Ossuary church at Sedlec.
Come Judgement Day, that is going to be one busy and confusing place.
You can find the official page of Sedlec ossuary at http://www.kostnice.cz/