This kept me wondering and made me realize of that 3rd dimension previously discussed with Joseph Grima. We are living over solid ground, and other than subways and rats, there is nothing but rocks and soil.
(See crust)Wouldn't it be great if you could find random sky-holes throughout the city just as we do subway entrances, only these lead not to a parallel position to where you stand, but to an upside down world where everything is foreign and undiscovered. Think Alice in Wonderland's rabbit-holes strategically placed along the city.