Green dystopias

I want to see a dense-packed dystopian urban setting... surrounded by incredibly lush, dense wilderness. As in, the reason that everything is piled up on top of everything else and people are living stacked like cords of wood isn't because they've destroyed everything and their cities have taken over the world like a bacterial culture, it's that the rest of the world is too damn poisonous and too fast-growing and too interested in cracking open your buggies and eating the nummy human interiors for anything to survive outside of these narrow strips of otherwise-dead land. (I imagine that'd be the way you'd answer the question of how you'd get enough resources in the first place to build a dense urban setting: you're in the equivalent of the Atacama or the Dry Valleys or something, only with bonus high concentrations of minable minerals.)

I have not thought through the logistics, here. I came up with this idea about two minutes ago.

In other news, I recently learned that the Sahara was a fertile region up until about 3000 BCE, and that is immensely cool.