Exactly how did the world "end?" I mean, at the beginning, when the builder (w/ the box) runs through the tunnel, we see that it appears to be in the middle of an earthquake (w/ rubble falling down from the ceiling).
Based on the later appearances of the giant moth & mole, I could only guess a nuclear war. That is, these creatures grew to enormous sizes due to radiation (a la the 50's flick, "Them").
The movie doesn't expand on this obviously. In fact, neither does the novel really. The details of what happened appear as a "what is likely to happen" in the the third novel: "The Prophet of Yonwood".
The US and another superpower are in political chaos and people fear the worst like they did during the Cold War. They mention repeated failures of diplomatic relations and both powers threatening to use their missiles to obliterate the other. The "prequel" really just tells the story of a girl who lives during the times. Eventually she moves out west where her father earned a job on a top secret project building an underground city.
At the end of the first novel: "The City of Ember", Doon and Lina find this woman's diary near the exit. When she was in her 50's she was selected to be one of the 25 adult females to go to Ember and raise two babies with a man. Obviously this war took decades to escalate to total annihilation.
Imagine a cold war standoff now but I with several countries in the standoff. In the books, it's never clear what the Builders were preparing for in terms of what state the world above would be in. Giant insects or mammals aren't in the book. Movie made them up.
Honorary Knight of Arendelle
Half-Blood #18 and Son of Poseidon, Son of Adam, Gryffindor 7th year
The second book explains albeit a bit unclear exactly. Aparently it was 3 wars and 4 plagues or 4 wars and 3 plagues..