What is a Charnel house?

from Wikipedia:
A Charnel house is a vault or building where corpses and/or bones are stored. They are often built near churches for depositing bones that are unearthed while digging graves. The term can also be used more generally as a description of a place filled with death and destruction.

Old World

Often, where ground suitable for burial was scarce, corpses would be allotted a certain period of temporary interment following death. This enabled the relics to be collected and the ground re-used for further burial. This especially occurs in particularly rocky or arid places.

Monastery of the Transfiguration (Saint Catherine's), Mt. Sinai

Saint Catherine's Monastery in the Sinai, is famous for having a working charnel house. Saint Catherine's was founded by Justinian in the early 6th century on the site of a prior establishment named for Helena of Constantinople founded about 313 A.D. The monastery comprises the whole Autonomous Church of Sinai, under the Patriarchate of Jerusalem. The site lies at the foot of Mount Sinai where legend has it Moses received the Ten Commandments.

Since the Sinai is an inhospitable place, the brethren of St. Catherine's have struggled to eke out a subsistence level existence. The difficulty in establishing a large cemetery in the rocky ground notwithstanding, relics are also gathered for temporal and spiritual reasons; a reminder to the monks of their impending death and fate in the hereafter. The Archbishop of Saint Catherine's is commonly the Abbot as well. After death, he is afforded the dignity of a special niche within the "Skull-House."

New World

A Charnel house is also a structure commonly seen in some Native American societies of the Eastern United States. Major examples would be the Hopewell cultures and Mississippian cultures. These houses were used specifically for mortuary services and although they required many more resources to build and maintain than a crypt, they were widely used. They offered privacy and shelter as well as enough workspace for mortuary proceedings. These precedings included cremation (in the included crematorium) as well as defleshing of the body before the cremation. Once the houses had served their purpose they were burned to the ground only to be covered by earth creating a sort of burial mound.