The inhabitants are made to believe that the world outside the silo is deadly and poisonous, which is true. When workers are sent out to clean the camera at the silo's surface, they are only able to survive for a few moments before ultimately suffocating and dying from the poisonous air.

Ultimately, Juliette is able to stay alive thanks to actually functional heat tape that keeps her suit airtight. And by getting over the hill she sees two very important things: The world up top is even more desolate and ruined than the cameras led them to believe, and there are many other silos.

Holston and Allison are actually dead.

Talking about that with Hugh, we came up with something called 'the syndrome'. It's a neuralgic response to the pressure of living in these conditions." His insight on "The Syndrome" implies that it could also be a mental health condition caused by the psychological distress of living in the underground city.