BT Telecoms Tower
BT Tower, scene of a bomb in 1971, and was an 'official secret'.
The BT Tower just north of London's West End was the first purpose-built tower to transmit high frequency radio waves, and it serves as a functional telecommunications centre designed to relay broadcast, Internet and telephone information around the world.
In the early hours of October 31, 1971, a bomb exploded on the 31st floor although no-one ever claimed responsibility for the device. No-one was injured in the blast but the damage took two years to repair.
Despite being one of most recognisable and conspicuous buildings in London, it was classed as an 'official secret' until fairly recently, and taking or possessing photos of the tower was technically an offence under the Official Secrets Act.