Page "Military citadels under London" Paragraph 17

At the northern end, the tunnel connects to a shaft up to the former Trafalgar Square tube station ( now merged with the Charing Cross station ), and to the BT deep level cable tunnels which were built under much of London during the Cold War.

At the southern end, an 8 ft ( 2. 4 m ) diameter extension ( Scheme 2845A ) connects to a shaft under Court 6 of the Treasury Building: this provided the protected route from the Cabinet War Room.

The tunnel was further extended ( Scheme 2845B ) to the Marsham Street Rotundas.