City of Ember

As an unspecified global catastrophe looms, an underground city known as Ember is constructed to shelter a large group of survivors. In addition, a small metal box intended for a future generation of Emberites is timed to open after 200 years. This box is entrusted to the Mayor of the City of Ember, and each mayor passes it on to his or her successor. Over time however, the significance of the box is forgotten and the succession is broken when the seventh mayor suddenly dies. The box opens by itself at the allotted time, but this goes unnoticed. Several decades later, Ember's generator begins to fail, while food, medicine and other necessities are in dangerously short supply.