Blast-proof and self-sufficient, the complex could accommodate up to 4,000 people in complete isolation from the outside world for up to three months. The underground city was equipped with all the facilities needed to survive: from hospitals, canteens, kitchens and laundries to storerooms for supplies, accommodation areas and offices. An underground lake and treatment plant could provide drinking water, and twelve tanks could store the fuel required to keep the four generators in the underground power station running for up to three months. The air within the complex could be kept at a constant humidity and heated to around . It was also equipped with the second largest telephone exchange in Britain, a BBC studio from which the PM could address the nation, and an internal Lamson Tube system that could relay messages throughout the complex, using compressed air.