In City of Ember, Chapter 3, how far underground are the Pipeworks Doon works in and what advice does his father give him about working there?

The Pipeworks are about fifty feet underground, with "tons of earth and rock and buildings" up above.  As he descends into the area withhis supervisor Arlin on his first day of work, Doon Harrow comes to this estimation by calculating that the distance from the river to the ceiling of the main tunnel must be "thirty feet or so", and that above that, the storerooms occupy a layer "at least twenty feet high".  Altogether, that comes to about fifty feet.

Doon had wanted to work in the Pipeworks because he knew that he would have access to the generator there.  Doon is aware that the generator, which provides electricity for the entire city of Ember, is failing, and he wants to fix it.  Knowing that he has been gifted with exceptional intelligence and mechanical ability, Doon had thought that patching up the machine would be an easy thing, and that just by looking at it, he would be able to figure out what to do.  When he finally sees the generator, however, he realizes that it is much more complicated than he could have ever envisioned; he has no idea how it works, and that there is no one who can teach him.  Frustrated and angry, Doon explains the situation to his father, who wisely tells him just "to pay attention" for now.  Mr. Harrow tells his son,

"You'll think of something.  You're a clever boy.  The main thing is to pay attention.  Pay close attention to everything, notice what no one else notices.  Then you'll know what no one else knows, and that's always useful" (Chapter 3).