Mission Cloacina was the codename of the pre-War bio-weapon project for a "Mutant Undermining Lifeform".
A mutated version of the common sewer rat was to be launched against the People's Republic of China. The rats would have been released covertly into an enemy's environment to undermine enemy infrastructure. Their aggressive qualities, dangerous hunger, and pervasive breeding would softening the target for ground invasion. At this point, the genetic kill-switch could be activated, allowing for easy reclamation – not including a significant, one-time clean up effort. Forced Evolutionary Virus testing had been rejected, as the results were sterile.
Behind the scenes
Cloacina (Latin, cloaca: "sewer" or "drain") is the Roman goddess who presided over the Cloaca Maxima ("Great Drain"), the main trunk of the system of sewers in Ancient Rome. She was originally derived from Etruscan mythology.
Mission Cloacina is mentioned only in Fallout 3.