Underground structures, including a hundred-year-old sewer, also presented problems during construction. The building is above the Bakerloo line of the London Underground: the Victoria line was tunnelled beneath in the 1960s, and presented problems for construction of the Egton Wing (see below). Noise from passing trains is audible within the radio theatre, but generally imperceptible in recordings. The ground floor was fitted with floor-to-ceiling windows overlooking the street, as the BBC believed that to finance such a project (costing £25 million in today's money) they would need to let the ground floor as a retail unit. The rapid expansion of the BBC meant this never occurred.