Bava Batra 102 - Investigating a Possible Forgotten Catacomb
If one finds an interred corpse, he must investigate further, to see if he is dealing with a forgotten graveyard, since the laws for a single corpse and a graveyard differ.
If one finds a corpse in an area not previously known to contain corpses, lying in its usual manner, he may remove it along with its surrounding earth, for it may be assumed that it is not a graveyard. If, however, he found three corpses and there is a space between them of from four to eight amot, it is considered a graveyard zone and the corpses may not be relocated. The upper limit of eight amot agrees with the opinion of Rabbi Shimon on the size of catacombs .
Art: Anton Alexander von Werner - Crown Prince Frederick by the corpse of General Douay at the Battle of Wissembourg