“Thousands of them have lived underground. They've lived there so long, there are only legends about people living anywhere else. Such a life requires rules. Strict rules. There are things that must not be discussed. Like going outside. Never mention you might like going outside.
Or you'll get what you wish for.”
Sheltered in underground silos to protect them from the toxic outside environment, people go their whole life without seeing the surface. Except for the cleaners, who mysteriously walk away after their task, to die.