There is a secret underground tunnel in Dublin's O'Connell Street - Apparently.
Built in 1828, the Dublin Sewer Tunnel is a 3.5 km long network of brick-lined tunnels that crisscross the city, including beneath O'Connell Street. The tunnel was constructed to alleviate the city's sewage problems and was designed by engineer John Rennie. Although it's no longer used for sewage, parts of the tunnel remain intact and are occasionally accessible to the public through guided tours.