Thirty stories below ground. Reinforced by twenty feet of the strongest materials ever created. It was so new, it hadn’t really gotten a name before the rush to build the underground city. It was fireproof, waterproof, earthquake resistant, rot resistant. Nothing could get through, nor could anything escape.
There were several hydroponics bays to supply air, food, and power. The lowest level of the city held multiple animals living on a surprisingly large grassy area. Artificial sun, artificial rain. Everything to remind the critters of a comfortable farm-like setting.
The Underworld could sustain a society of ten thousand people, possibly more. Six thousand went in for now. Five thousand, six hundred children and young adults and four hundred “responsible” adults would work together to rebuild what was once North America. There was no longer Canada, United States, or Mexico. The countries were finally one. Perhaps it was the only good from the end of the world.
Sixty years from now, The Underworld and the other twelve facilities around the globe like it would open and those children–now grandparents or even great-grand parents–would rebuild the earth. For now, they were trapped in a self-sustaining tomb.