The corpses of all the men he killed were buried in this hallway resting peacefully.  Joe was a hit man, and his job was to kill people and he was good at it.  He figured that everyone eventually dies, and some people just need a little help doing that and besides the money was good as long as nobody knew where the bodies were buried.  Joe wanted the bodies to be unnoticeable, and he thought that could be accomplished by having them in a visible setting that would mask their presence, so he hid them in plain sight.  The optical illusion of the dip created in the floor was obvious for anyone who walked down this hallway, as nobody would ever miss it.  Most people were happy to get out of this hallway without losing their balance and nobody wanted to stick around here.  Since the dead bodies blended in…