Board Thread:General Discussion/@comment-37807741-20190522182143/@comment-5956954-20190523013136

Catacombs: An underground system of tunnels and chambers with recesses for graves, used (in former times) as a cemetery; a subterranean tunnel system used for burying the dead, as in Paris or Ancient Rome.

In other words, it's just what the ruins is; an underground system of tunnels and chambers that has been lost to time.