Follow this link and read the lavishly-illustrated account from a band of Barcelona track-runners who went all Forsake-the-Sky and spent their weekends exploring deeper and deeper into the underground warren.
Expeditions into the older, deeper cavities of a city's foundations never end well. There are ancient buried horrors which should remain buried, while the claustrophobic darkness is not good for the psyche. There are documentaries about it.
Anyway our tunnel-rats rapelled down 80-metre ventilation shafts and hiked in darkness for kilometers and hid from jigger-riding guards, and at the end of it, they found an abandoned Tunnel Boring Machine, IN ACCORDANCE WITH THE PROPHECY. I am not making this up.
extending the topological complexity of the Barcelona metro to a level where it becomes the functional equivalent of the Gödel-Mandelbrot Algorithm, and OPENING THE GATEWAY. Then the final act would become a desperate struggle for survival against the cannibalistic mutants.
IRL, however, I can only say WOW.