h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Prague is without doubt one of the most beautiful cities in Europe and full of life, beer, music and puppets.
However, for one of the most macabre sights you'll ever see, take a day trip to Kutna Hora and look for the Gothic chapel with the incredible underground ossuary. The tens of thousands of skeletal remains have been creatively displayed on every available surface. A huge bony chandelier hangs from the ceiling, four enormous bells of bones fill each corner of the crypt and the Schwarzenberg coat of arms you'll see all over Bohemia takes on a whole new perspective. All because some medieval cleric claimed to have sprinkled the graveyard with holy soil from Golgotha creating a hugely popular final resting place and a surplus of skeletons over the centuries that followed.
The town also has an old silver mine and a nice cathedral making a highly recommended day trip if you can work out the bus timetables.

For a similar sort of outing, but with excellent wine included, go to Melnik. There is an ossuary there as well, but the bones are placed in very interesting and esthetically pleasing patterns (1 wall has the skulls arranged in the shape of a heart - leg bones elsewhere) (1 wall spells INRI) (1 Wall has a cross hatched pattern) (The last wall you can walk through).

Also, Melnik is know for its wine. Very excellent. And, it is on a hill overlooking a river. Quite beautiful. Oh, and when you are in the ossuary, the play really eiry music.

So who do the bones belong to? Monks or something?

Monks, priests, nobility, peasants with enough money to afford a decent burial, anybody and everybody. Seems to be a central European thing to collect bones rather than just fill holes in the ground. I vaguely remember seeing a subterranean tunnel with walls full of bones underneath the cathedral in Vienna as well. However it seems to be just a Czech thing to get creative with them!!

I think what happens is that the physical space in the cemetary is limited. And, you have to figure that after about 100 yrs, no one will visit the grave anymore (graveyards really are there for the living after all). So.... no one would mind if you dug the body up to make room for a more recently deceased person. But... you would still want the body to be on "holy" ground, so... you would stick the bones in the basement. Easy solution. If you ever look around graveyards in Europe (with the exception of the Jewish cemetary in Prague - a site to see) you will notice that the dates don't go farther back that 150 yrs. Makes sense.

Also, There is an ossuary beneath the main catherdral in Vienna - and you can tell that some of the monks were conciences with the remains and some just tossed them in, but the ones in the Czech Republic are beautiful.