In general, catacombs (meaning "by the tombs") are man-made subterranean burial chambers that house the dead of a family. These chambers are connected by narrow passageways. Inside the chambers there are niches in which family members are laid to rest, or the entire chamber is devoted to the entombment. While some catacombs were decorated, they all were affected by the dampness of the underground.
It is ironic that Montresor remarks upon this dampness, but at the same time he cautions Fortunato about the niter on the walls.
"Pass your hand," I said, "over the wall; you cannot help feeling the niter. Indeed it is very damp...."
For, niter, which is potassium nitrate, and is found as "an efflorescence in hot, dry regions." (It is also known as saltpeter.) While niter does form on cave walls, it is not usually associated with dampness.
This contradictory statement of Montresor is in keeping with his use of reverse psychology and the perverseness of his relationship with Fortunato--who is anything but fortunate. In addition, Montresor's contradictory statement is in keeping with the motif of double meanings as throughout the narrative Montresor forms double entendres on such things as his protests that Fortunato is a "man to be missed," the trowel and his being a "mason," "For the love of God," and "In pace requescat."