The City and the Stars   by Arthur C. Clarke

Mankind has encased himself in a sealed city, except that every million years a soul is let loose to test the water and see if it is safe to go out. The book follows the unexpected adventures of such an individual, who eventually leads mankind on to its next stage of development.

In the meantime the story of how man entombed himself in the first place unfolds, with never-ending twists and turns that keep the whole thing totally unpredictable.

The slight reservation I have is that the book bandies about the ideas that man lives in the entombed world for many aeons, millions of years, and even eventually suggests a time-span of a billion years. It is hard to believe that things would last so long (unimaginable amount of time) without some intervening catastrophe or evolutionary step (althouh the book does go a long, long way towards explaining this -- it is all a planned part of man's well-designed exile from the wider universe). Ultimately you have to remember that the book was written in 1965, when expectations were different; I think if it had been written today that the time-span would have ben scaled down to some ten thousand years tops.

Then again, if I stop to imagine the world a billion years from now, I don't think I could imagine it any better than Arthur Clarke does!