Well, why don't you pack up the fams and head over to the Alton Towers amusement park in Staffordshire and find out! That's right, hide your friendly neighborhood Scottish constable, Nigel; The Wicker Man is coming to Alton Towers just in time for Beltane!
When I read RELIQUARY, a novel in which a prehistoric monster was terrorizing the tunnels under New York City, I noticed in the author's notes that her reference to an entire underground society in the New York City tunnels was absolutely based on truth.
A tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.
80 different countries have now formed an alliance against the Old World Order / Nazi-esque cabal. Two huge underground cities were destroyed on August 23rd -- which may be seen by historians as the critical moment in which the war was won.
Secret Underground Bases and Facilities. The history of humanity is laced with stories about secret underground facilities and bases that go back to the beginning and - more recently highlighted with catacombs of many cities - and caves that lead to underground chambers for all sorts of purposes, mostly thought of as clandestine - run by the men in power in a particular timeline.