Beginning with a mishnah on 100 -- about the structure of catacombs: family burial places carved into the recesses of a wall, and the specifics of the dimensions required for them. The Gemara delves into the spacing of niches for burial coming off chambers, including what direction to place the body. Also, the question of coming across a buried body - or two, and how to ascertain whether they are buried in the way of Jewish burial. When it comes to three bodies buried in that way, the question of moving the bodies for reburial is more complicated.