today's fact is this:
the BT Tower, in central london (this one: http://upload.wikimedia.org/wikipedia/commons/1/11/BT_Tower_2004.jpg) was officially a 'secret' until the mid 1990s. it did not appear on OS maps, and if you took a photo of it you would be breaking the Official Secrets Act. y'know. despite the fact that it towers 175m over central london and can be seen from miles away. it's existence was "officially" confirmed by an MP in 1993 (my MP, actually) in a Commons debate as an example on trivial information being kept nominally 'secret'.