It was lunchtime on London's Northern line. Deep underground, passengers were getting on and off the tube trains as normal. Two men boarded a train at Colliers Wood in south London. As the train gathered speed towards its next stop, Tooting Broadway, one of them got up from his seat and dropped a small carton of face powder out of the window. He could have been idly throwing away litter.
As the train sped on, the carton hit the tracks and burst. Out spewed millions of tiny spores, which began to spread throughout the dark tunnels. Dust swabs taken after three days and two weeks showed that the spores had spread as far up the line as Camden Town station in north London, 10 miles away.
This really happened. But the two men weren't terrorists but government scientists. And the spores weren't anthrax spores, but a harmless micro-organism designed to mimic clandestine sabotage with anthrax. This was an official experiment in 1963, and it showed how easily saboteurs could inflict a potentially devastating attack on Britain's capital.
I promise never to be bored on the Northern Line again.