At this point, I'm not gonna answer any "How did they..." or "Why is that..." questions. Just suspend disbelief already, dammit.
TRANSCRIPTION COURTESY OF JOSEPH HOUK - [Exterior shot of a "non-descript office building in York, Pennsylvania"] (CW) "I must say they've disguised this well!" (Carmen Scott) "Yep... A generic-looking Federal office building on the outskirts of York, Pennsylvania. Looks like it could be for anything... (Train depot hallway, with KS, AJ and CW) ...But what it really is, is a station for the subway to Paratopia." (KS) "The area was originally a huge natural gas well owned by the government. (Downbound train) Once the gas was depleted, in 1965 a cave explorer went in and marveled at how vast and majestic the caverns were." (CS) "After determining the feasibility of putting a massive resort many miles underground, a consortium of seven nations took it over and built it up." (KS) "It's a nice little ecosystem, powered and cooled by deep ocean water. Other gas wells help with power and heat, and artificial sunlight sustains plant life." (CS) "The caverns are cool enough, but there's also stuff like rainforests, lakes, a theme park and a huge beach!" (CW, looking around after they disembark from the train) "Wow... I like it already!" (KS) "Yes, you'll find out that Paratopia is full of surprises... [They're greeted by an old guy in a multi-colored shirt] ...Starting with your host, retired director of operations D.B. Cooper!" (DBC, as Cassie is speechless) "Welcome to Paratopia, a piece of heaven under Earth!"