Its core was the dark, heavily shielded cylinder that had sheltered the earliest settlers: desperate pioneers, driven to mine the Rings of Saturn despite their sleets of radiation and complex electrical storms. The central core of Goldreich-Tremaine was as dark as a nut, a stubborn acorn that had endured and broken forth at last into fantastic growth. Hubbed spheres wheeled about it, radar installations slid with sleek precision on external tracks, two huge tubed suburbs turned in counterbalanced array on white ceramic stems. And all about the inner complex was a lacy network of habitats in free-fall. Outside the bubbled suburbs—the "subbles"—loomed the immaterial walls of the Bottle.