Once upon a time, there were over 7 billion humans, but that was a long time ago.  Tensions had come to a head, and the world faced a global nuclear crisis.  Our leaders decided to build vast underground tunnels, connected to bunkers designed to be lived in for years.  These bunkers, it was said, were very livable, too, containing windows that stretched from the floor to the ceiling, plenty of food, and the most comfortable furniture available.  Judging from the fact that none of them were still around; I would guess that their plans didn't work.  This is, of course, rough details and speculation, as you can tell.  History was always my worst subject, so let's get away from it, and talk about the now.
In the now, as there always has been, there is a school.  All any of us can remember was the school.  None of us knew when the school was built, or cared about how many years it had been.  We had no way of keeping track of the date, and no real reason to.  The school is a lot like what the very first government must have been like.  We were allowed to leave the school any time we wanted, but we all knew that was stupid.  No one that left the safety of the school ever came back.  The radiation in the world didn't do what the fiction of the before-times told us it would.  As far as we knew, no animal could survive the jungle that was the outside.  The reason that the school survived and was still perfectly fine to be lived in, was a mystery to us.
We all knew about the sealed door in the basement.  The teachers were certain that it led into the underground tunnels of legend.  The smaller ones were fond of making up stories about what monsters must lurk deep in the underground passages, while the bigger ones talked about the Promised Land that they must lead to.  I never took stock in any of the stories.  The stories that I liked weren't about any horrible monsters, legendary heroes that survived the nukes, or paradise past so many trials. Such things simply didn't exist.
Then, one day, he appeared, and changed everything.  The door was a favorite meeting place of the little ones after our studies had ended.  It was just after sunset, when they heard a loud clang from behind the door.  They ran up to the rest of us, speaking of a terrifying noise.  None of us really believed that such a thing had happened, but we knew that they wouldn't calm down unless we went to check it out.  Once we arrived at the door, we found an unfamiliar man collapsed unconscious next to it.  We took him to the infirmary, where he soon came to.
When he awoke, he asked where he was.  The best answer we could give was "The nurse's office of a school."  None of us knew what was safe to tell him, since nothing like this had ever happened before.  He told us that he'd been searching through those tunnels for what felt like an age.  He showed us the keycard that he had slotted into the door to get it to open.  Aside from that, he said that all he remembered was his name: Jack.
Naturally, the smaller ones were very eager to hear what he had to tell us about the tunnels, but we could tell that he needed some rest.  Every day that he stayed with us, he said that he felt a little better.  It wasn't long before he was ready to leave again.  At that point, he asked if any of us wanted to come with him.  Before anyone could volunteer, one of our teachers came forward and offered him a map (I always thought he had been hiding something).  He said that, if we were to follow it to its eventual end, we might find a better place to live.  Before any of us had a chance to object, we were all simply drafted into this potentially deadly mission.

I could tell that many of the little ones were disappointed in the tunnels.  Their tales of terrifying, monster infested caverns holding great treasure proved to be very much incorrect.  Instead, what we found was probably closer to the service tunnels and metros of old. I’m no architect, but even I could tell that these were not particularly well designed for any sort of major disaster.  If these tunnels were really supposed to be the legendary tunnels our leaders built for the safety of their people, then I could understand why seemingly everyone else was dead.  Living in dilapidated tunnels, though, was far more appealing idea to me than dying in nuclear fire.
For what must have been weeks, Jack led us through the tunnels, following the map to our salvation.  Though the smaller ones were disappointed in the tunnels, the bigger ones were thrilled at the prospect of their amazing fantasies of utopia being potentially just around the next corner.  Many of us soon became skeptical that this map that we were following, held by this man we knew next-to-nothing about, was actually supposed to be a map of these tunnels we were exploring.  Several people insisted that the map wasn't even a map at all, just a piece of paper.  No one was allowed to look at the map other than Jack.  He seemed to never sleep, and never took it out of his sight.  Whenever someone would complain, he would insist that we were "almost there," that "the end was just minutes away."  As morale lowered, more and more people insisted on seeing the map.  Just as it seemed that we were ready to mutiny against our expedition leader, we found it.
What was it?  A door, just like ours, with a card slot just like ours.  Our hopes were dashed at such a sight, and this was the final straw for us.  We grabbed Jack, wrenching the card and map from his hands.  Our exhausting journey was for naught, our lives would all go back to what they were.  Then, as we opened the door, we realized something.  The way that it was described in the books, our school was one of the bunkers of legend.  As we gazed out into the vast jungle, we wondered how many more of these schools there must be.  As life returned to normal for us, we sent Jack away, back into the tunnels.  No one has heard from him since.  Yet the school survives, and will always survive, just as many of us hoped the others did, too...even though we knew better.  Paradise, along with whatever might have taken us there, was gone, just like everything else from the old world.