I often have to remind myself to look up when I'm exploring a city. But sometimes a city's best kept secrets are hidden underground. For two years I was living right above an important part of London history: the deep-level shelters at Clapham South. Built during the Second World War to protect the people from the German bombings. What I didn't know was that a small group of people gets the chance to visit them today. The London Transport Museum organizes the Hidden London tours a couple of times a year. They sell out quickly so when I read about this year's tours months ago I signed up immediately. When the day finally arrives I'm super excited....