LONDON — From Narnia to Alice in Wonderland, Britain’s best adventures begin with hidden doors or secret tunnels — and this is a tale with both. Behind an unmarked blue door in London’s Chancery Lane, the historic legal district where Charles Dickens once worked as a clerk, CNN puts on a hard hat and high-viz as we embark on an exclusive hour-long tour 30 meters below ground (nearly 100 feet). We’re here to explore a mile-long series of tunnels so clandestine that until 2007 they were protected by the UK’s Official Secrets Act. There’s a literary connection here too, as...