“Not many travel guides mention these underground systems, creepy tunnels and enormous caverns, located sometimes right under our feet. Most were originally classified (no doubt by Sector Seven), and only recently open to the public. Some are still impossible to enter (due to blocked access points) and hard to explore in their entirety (due to the lost or confusing documentation). But this is where the “spirit of adventure” comes in, as multitudes of amateur photographers descend into the unknown to bring back evidence of things unseen.” (Abandoned Tunnels & Vast Underground Spaces)
