At the end of Jeanne DuPrau's novel The City of Ember, Doon and Lina emerge out of the Pipeworks and find a very strange and different world to the one they have been used to. They see stars and crickets and the sun for the first time and begin to venture out into this new world. However, it isn't until they read the book that they brought with them, which contains journal entries from one of the original settlers of Ember, that they realize that this new world is the one that their ancestors came from and that the Pipeworks they have traveled through are the same ones the first settlers entered to reach Ember nearly 250 years ago.
As Doon and Lina continue to explore the new world, they come upon a crack in the ground that leads to a narrow cave. When they enter, they come to a cliff, and looking over the edge, they see a shocking sight.
They looked out into a cave so enormous that it seemed almost as big as the world outside. Far down at the bottom shone a cluster of lights.
"It’s Ember," Lina whispered ...."Oh, our city, Doon. Our city is at the bottom of a hole!" She gazed down through the gulf, and all of what she had believed about the world began to slowly break apart.
As incomprehensible as it is to Doon and Lina, in seeing their city from above, they realize that they have lived their entire lives not just in a separate world, but underground. They realize that there is a surface to the world with a sky above it, that this is where they have escaped to, and that Ember and all its citizens have lived for hundreds of years below the surface of the Earth. They also realize that this is where their ancestors always meant for them to come back to. It is time for the people of Ember to return to the surface world. It is with this knowledge that they send their message down to Ember and beckon their people back into the light.