Why was the city of Ember created in the book entitled The City of Ember?Nothing - I just need the answers please.

The city of Ember was created in the aftermath of an
apocalypse, presumably a nuclear war. The war had rendered conditions on the face of the
earth unlivable, so Ember was built far beneath the ground to allow survivors of the war
to continue to thrive. Ember is the sole refuge remaining for the human race. Because it
is located so far beneath the earth's surface, it is completely dark, and it is
sustained by a complicated and ingenious system run by electricity. At the time of the
story, over two-hundred years after the Ember's inception, the city is threatened
because it appears that the electrical system is about to give out. If it fails
completely, the city will be plunged back into complete
darkness.

Ember was not intended to last forever. Its
creators had estimated that, after a certain amount of time, conditions on the earth
would improve to where it would be able to sustain life once again. These founders had
written instructions as to how the people could escape from Ember when that time came,
and the instructions had been placed in a box which was to have been passed down from
mayor to mayor of the city. Unfortunately, after a number of years, the box had been
lost, and ultimately forgotten. Now that the electricity system is threatened, the
people are stuck in the doomed city with no way of escape.