Iapetus Ocean, an ancient ocean between the paleocontinents Laurentia and Baltica
It is named after Giovanni Cassini, the discoverer of Iapetus; 'Regio' is a term used in planetary geology for a large area that is strongly differentiated in colour or albedo from its surroundings.
During the months that Colin's crew in Birhat labor to get the Imperial Guard up and running, the scouts duel the Earth forces, hurling asteroid after asteroid at the shield while whittling down the fortresses and ships, all in preparation for their final blow: hurling the entire moon of Iapetus down the gravity well of Sol at high speed, and aimed directly at Earth.