The planned course of the Picc-Vic tunnel would not follow a direct route across the city centre but instead would skirt around the south of the city centre before curving north towards Victoria. An obstacle to the construction of the line was the subterranean Guardian telephone exchange, a subterranean structure that extended beneath the city centre south of Piccadilly Gardens. The exchange had been designated as a nuclear bunker, and while SELNEC planners were aware of this structure, they were unable to mention it publicly as they were forced to sign the Official Secrets Act in 1971.