Mercury arc valves were used extensively in DC power systems on London Underground,[13] and two were still observed to be in operation in 2000 at the disused deep-level air-raid shelter at Belsize Park.[14] After they were no longer needed as shelters, Belsize Park and several other deep shelters were used as secure storage, particularly for music and television archives. This led to the mercury-arc rectifier at the Goodge Street shelter featuring in an early episode of Doctor Who as an alien brain, cast for its "eerie glow".[15]