What is a summary for chapter 20 city of ember the first book ember?

In Chapter 20 of "City of Ember," Lina and Doon finally find the "Secret of the City." After a long and dangerous journey, they reach the bottom of the city, where they discover a hidden room filled with instructions for a power source and a map leading to a "way out" of the city. They realize the city wasn't always doomed, but that someone intentionally sabotaged it, leaving its inhabitants to live in perpetual darkness and dwindling resources. The chapter ends with Lina and Doon feeling a surge of hope, determined to use the information they found to bring light back to Ember.