Mysterious passage to new lands

I was walking down the weak flow of rainwater in the underbelly of the city, having one-sided conversations with imaginary characters as long and twisted as the paths I was following. There was a warm and misty ambiance that morning and the playful swirls of vapor through the artificial light truly emphasized the eerie dreaminess of that moment. Whilst breaching through a crack in the monumental concrete pipe into a shallow passage I found myself explaining to my fictional oratory my liking for these long walks through the drains in a very satisfactory and comprehensive way. It went something like this: It is my belief that such an environment disrupts our most intrinsic expectations. The inevitable shadow of imminent social interaction, the ever-repeating diurnal cycle of routines and obligations, and the incessant flux of sensory stimuli, all are gone. With such fundamental constants ripped out of our consciousness all that is left is the pure, emotionless void of existence and unconstrained flow of imagination. A simple state of uneventful yet thrilling wonder that seduced me so long ago and is probably common to other exploration practices and meditation.
The crack I had entered only a few seconds ago was yet unknown to me and lead me deeper at a shallow angle. Even though the opening was mostly of natural origin I could clearly tell that the passage had been artificially enlarged to human proportions by the way of hand tools. Interestingly the pickaxe markings indicated that the passage had been dug in the opposite direction I was walking and in a far anterior time to the drains I had come from. I stopped, slowly turning around to face the entrance. I could not understand how an older path could have emerged into relatively recent storm drains. Walking back a dozen meters I watched closely the crumbling section that preceded the concrete end. This part was also man-made, I thought, yet seemed to have been cleared out rather than forcefully excavated.
Upon inspecting the entrance again I came to the conclusion that the crack must have been created from the outside by someone wanting to access the long-buried tunnel. They must have then cleared away the couple of meters of gravel that separated the ancient quarry from the pipe.
Yet a person with such knowledge of the undergrounds and dedication is quite uncommon and chills ran down my back despite having no apparent reason to worry. Surprised by my initial lack of discernment I had by that time completely snapped back from my mental divagations and rushed into the mysterious opening with a renewed interest.
As I progressed in the uneven yet sturdy gallery I was conflicted as to the nature of it. It couldn’t be very recent at all judging by the ruggedness of the carving and the beautiful concretions lining the upper walls. I had heard of Roman salt mines in the area but I would have never expected one this expansive. As more and more intersections followed I was forced to start carefully mapping the explored areas. With a notebook in one hand and my torchlight in the other, I slowly drew what looked like fish bones emerging from a central spine with some snaking interconnections between them. That main axis I had not fully explored yet but I expected it to run for a couple miles giving birth to even more sub-sections like the one I had come from. I also expected to find the main entrance at the end of the spine like it is usually found in more modern quarries. I spent some time sketching out the entire bowels of the antique beast which now had slowly turned into an enormous sea serpent as the spine kept going indefinitely in an organic wave. The volume of that central tunnel was truly astounding for its time, almost reaching the size of modern excavations and the tall jagged walls were gloriously glowing like broken glass from the overgrown salt crystals in the bright white beam.