In Europe, churches that were established over a thousand years ago are still being used. It's all well and good to want to be buried in the church cemetery when your time comes, but there's only so much space. That's why in many older churches, graves are opened after a certain number of years so the space can be re-used. Then there are mass burials that are made necessary by epidemics, in which many bodies are buried together, with their anonymous bones retrieved much later.
The bones, which are all that's left at that point, are interred somewhere else where they take up less space. Some places have extensive catacombs for bone storage; other churches use them as interior decoration! And why not- they have to go somewhere, and they remind parishioners of their mortality. Get a look at ten such churches, from Italy to Austria, from Portugal to Poland, at Scribol.
(Image credit: Merlin)