These underground tunnels have been described as DUMBS, Deep Under Military Bases. A few years ago I read that 'aliens' provided advanced technology (huge cylindrical machines) were used to create these cities across the world. Claims that they could dig miles of tunnels in a very short space of time, were made. Access was said to be through Area 51, the top-secret military base in the USA.
I also read that the people living there were soul-twins and each human on the surface had a twin underground to replenish the population if it was every destroyed. These advanced humans communicated via telepathy.