It have the bones of 40-50,000 people arranged inside. The cemitery history beggin in 1278, when a abbot was sended by the Bohemian king to the Holly Land, and returned with a handfull of Golgotha earth that he sprikled over the abbey cemitery - this made the place a very desirable place to be buried.
During the Black Plague in the 14th century and the Hussite Wars in the 15th, the population of the cemitery increased even more - around 1400 a church was build in the middle of the cemitery, and the lower chapel used as an ossuary.
In 1511 the task of exhumating skeletons and stacking their bones started - but the decoration we see today was only done in in 1870 when František Rint, a woodcarver, was employed by the Schwarzenberg family to put the bone heaps into order.
And he did in a sort of artistic way.
I recon to saw this place at first in "Ripley's Believe it or Not" in the 80's, and never forgot it - I had the chance to finally visit it last year.
It is a little smaller than what I envisioned, and no tripods are allowed, had some light and as I was the first one on the morning this place was all mine to photograph without people around (it is a popular place this days). My Nikon D610 handled well the situation along with a 24-70mm f/2.8 lens.