I'm trying to find the title of a book I read as a kid, probably somewhere in late elementary days. I read this one around the same time as The White Mountains series by John Christopher, ~30 years ago, which I would also recommend to today's YA generation. Pretty creative stuff.
It's about a boy whose father has a secret underground garden because it's some sort of post-nuclear age where all the official food comes out of something like vending machines. The boy's father disappears and he goes on a search for him, and through a mysterious series of events ends up finding the last people on Earth strapping themselves into an airplane or spaceship to go into suspended animation until the Earth renews itself.