A network of secret tunnels hidden under central London for decades will soon be opened up to the public under an ambitious £220 million plan for a huge new tourist attraction “as iconic as the London Eye” unveiled today.The Kingsway Exchange tunnels, an expanse of 8,000 sq m of passageways several hundred feet below High Holborn, were shrouded in mystery for most of the 20th century with details covered by the government’s Official Secrets Act, because of their wartime role as a base for MI6 officials.