Visible from virtually everywhere in central London, the 189m-tall BT Tower was the highest structure in the city when it opened in 1966 (St Paul's Cathedral was the second-highest at the time). It was closed to the public in the early 1980s because of terrorist threats. Incongruously for such a conspicuous structure, the building was officially a ‘secret’ and didn’t appear on official maps until 1993, when Member of Parliament Kate Hoey used her parliamentary privilege to ‘confirm’ its existence.

The tower is still a major communications hub and doubles as an air-pollution monitor. It is also a listed building, which meant getting special permission to remove the defunct circular antennas in 2011. It is occasionally opened to the public for one day as part of the Open House London, when lucky ballot winners can secure tickets for a guided tour.