I actually remember two stories something like this, that I read in elementary school. They were in those Mifflin Readers, or whatever they were called. I think one of them was called "Galaxies" and the other was "Serendipity"?
My memory is vague, but I remember both stories involved a boy and a girl, living in a future, underground society. They were told that the outside world had long since been poisoned, and was uninhabitable. In one of the stories, it ended with the two kids coming out of a cave, and seeing a lush valley with plenty of trees and greenery, and the world had pretty much healed itself. In the other story, the two kids came to a sealed door at the top of a long ladder, or something like that. They could hear something scurrying, on the other side of the door, and were excited at the prospect of a healthy, pristine world out there waiting for them. But when they got the door opened, they discovered nothing but heaping piles of garbage, a polluted world, and the scurrying sound they heard was that of rats.
I have no idea what these stories were named. I do remember the first story, with the optimistic ending, had some illustrations that were done in a sort of trippy, psychedelic 70's cartoon fashion, but the second, more downbeat story, was illustrated in a more realistic, dark fashion.
I'd really be curious to re-read these stories again. Hopefully they're still out there, somewhere.