The City of Ember: Chapter 8

In chapter eight, Explorations, they talk about what Doon did at his job. He finished early, so he decided to look around. He discovered a few bugs that he though was interesting, and then drew them... things like that. He also explored the tunnels. One day, he found a roped off door that read: CAVED IN, NO ENTRY. But he went in anyway, and found out it wasn't a roped off tunnel at all, it was just a big dark tunnel, leading to a locked door. Doon then wanted to know about a movable light to use to see in the dark, so he went to the Library to find out more about fire. He was struggling with his search, and just as he was going to give up, Lina ran into the Library.

QUESTIONS: What do you think the locked door leads to? An escape? Do you think Doon will eventually find a movable light? What do you think Lina wanted?