Sometimes it is hard to tell exactly what environment they are in, because the Underground is literally a cave system. There are openings in the caves, as well as grass and trees in some areas. It is vary much like a prison, with cells and wings. As for the population. There are a lot the beginning of the month, then the population kills off to a constant average of a few hundred. In these 1st few posts, it will be more crowded, but as the games approach, it will become less crouded.