Tunnels are the millions of miles of subterranean passages beneath every city, state, country, and continent
Denver airport is one known opening, as well as the underground facilities carved deep into the Colorado Rockies and down below sea level. Natural caves also make up parts of the maze-like system, such as Mammoth Cave, the Bell Witch Cave, or Sauerkraut Cave, all in Kentucky and Tennessee. There are more than 3,000 miles of tunnels beneath Montreal alone!
These tunnels are terrifying and dark, driving some to blindness or madness, and resulting in many Legends and superstitions surrounding them, such as the need to hold one's breath for the duration of the tunnel so the beasties and monsters will not hear and consume you.