The vaults are very damp and encrusted with niter, a crystalline salt used to make gunpowder, as a fertilizer, and in medicine. They are also very large because this is where the dead bodies of family members are laid. Montresor reminds Fortunato that his family is very large. As they are walking, Montresor tells Fortunato to "observe the white web-work which gleams from these cavern walls." He continues, saying, "We passed through walls of piled bones, with casks and puncheons intermingling." Casks and puncheons were used to store the wine, so the catacombs have several uses. "We are below the river's bed. The drops of moisture trickle among the bones." Not only are the catacombs very large, they go down very deep below the surface. "We passed through a range of low arches, descended, passed on, and descending again, arrived at a deep crypt, in which the foulness of the air." As they go further down, the smell of the air changes and smells bad. We are reminded that the Montresors do punish those who offend them.