Just a little something that I had as a forum game, I'm interested to see where this may end up :)
When the threat of total destruction of the human race first occurred, the Davenport Technology Group decided to do their part to preserve the world. They were the largest company in the world at that time, and each of their buildings was designed with impenetrable, self-sustaining vaults underneath the ground. They offered their employees and their families sanctuary during the war and afterward. Each continent held three buildings, and each building held a hundred families.
By the time in which the earth was livable again, Davenport Technology Group was the only remaining mark of mankind on the planet. Centuries had passed, and the families had intermingled and learned how to live with the land, not abusing it. The culture was peaceful, and thriving. Many people were afraid to venture to the surface, but eventually they started living above ground again.