Cellblock was a cut multiplayer map in Call of Duty 4: Modern Warfare.
All that is known to prove its existence is a file called "compass_map_mp_cellblock.iwi" present in the game's source files. In addition, its name is known because it is referenced in the game's mod tools in the file "mpui.str" in the raw/English/localizedstrings directory.
From the name, it can be inferred that the map is of a jail cell block in the Middle East. From the minimap, the map appears to be small, centered around the main building which is implied to be a jail. The jail appears to have three stories. On the outside there are other structures of unknown purpose, as well as stacks of barrels and other generic objects and a truck that has crashed through the front wall of the jail.
The map shares similarities with the map Vacant.