The Command Center (Commonly abbreviated as CC) is the most important Terran building. Terran players start the game with one Command Center. It produces SCVs, the worker unit, and is at the root of the Terran Tech tree. It is also the Terran resource gathering facility, and without it the Terran player cannot gather resources. Command Centers can have one of two add-ons, either a Comsat Station (unlocked by the Academy) or a Nuclear Silo (unlocked by the Covert Ops). The Command Center can be lifted and flown around, albeit very slowly. One of the defining elements of the Terran race is that Command Centers can be built inside a Terran's main base, usually above the ramp on high ground, and then floated down to expansion(s) to make it easier to reach them.