Mu 3, R'lyeh: a young, watery super-earth, whose rocky core is rich in fissile uranium isotopes and is submerged under thousands of kilometers of ocean. There is no dry land, but fissile isotopes regularly bubble to the surface from undersea vents, undergoing prompt criticality as they hit submerged sheets of exotic deuterium-rich ice. These "blue smokers" can be, and are, harvested for their uranium salts, which provide for easy spaceflight and electrical power generation.
The biosphere comprises largely of Earth-like organisms seeded by the first colonists untold ages ago to provide a basis for a sustainable food chain and aquaculture source to fix nitrogen, produce hydrocarbons, and otherwise make the world habitable for those humans willing to spend their lives on vast floating semi-organic mats.
Leng, the sole terraformed moon of Mu 3, is a low-gravity, low-resource place that is very marginal for production. Originally intended as a support station for the fissile mining industries on R'lyeh, it expanded after the Crash to become a world in its own right, although it is reliant on imports of uranium, nitrates, and foodstuffs from R'lyeh; in return it provides a manufacturing base.
Mu 3 is the homeworld of the Commonwealth of R'lyeh-Leng.