Venus

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Earth's closest neighbor is far too hot and crushing for humans to survive on its surface. The atmosphere protects against radiation, which is nice, I guess.

Skystead

In 2081, the space program of China's Guangdong Economic Zone dropped a probe into Venus's atmosphere, with the goal of using nanotechnology and the compounds in the atmosphere to chemically construct a floating platform. The experiment succeeded; after about two years of growth, the platform was stable and, being less dense than the Venusian air, floated at about 60 km above the surface, above the crushing depths of the atmosphere. In the mid 2080's, several more seeds were dropped around the planet. These "skysteads", as they came to be known, formed the basis for a habitable Venus—though still hotter and denser than any environment on Earth, the skysteads bring Venus within the realm of survivability. A few of the skysteads were placed at the poles, where they get permanent twilight sunset, and became a tourist destination. Hydrogen and oxygen can be chemically separated from the compounds it normally resides in, and plants can use this produced water along with the abundant CO2 in the atmosphere and plentiful sunlight.
The skysteads themselves are partly constructed of sulfur compounds, and have an effect of sequestering sulfur out of the atmosphere. By the time of the Artifact Wars, Venus is covered in fractal, cloud-like floating skysteads, and the atmosphere is largely non-toxic to humans (though a breathing mask is still required to avoid CO2 poisoning).
Skysteads are located at slightly different altitudes, which are actually measured as depth into the atmosphere using fathoms (about 6 feet) as the main unit. This measurement standard begins at zero fathoms being at 62 km altitude (the highest altitude at which a skystead can theoretically be stable) and increasing as one moves towards the surface. Most skysteads are at between 800 and 2000 fathoms (58–61 km altitude). One small skystead, a tourist attraction at Venus's north pole, is named Negative Fathoms—it uses jet engines to hover above the theoretical "fathom line", where the environment is more comfortable for non-natives.
Venus's colonist population is mostly from China (who launched the nanotech probe) and Australia (who invented the technology and licensed it to the Chinese in exchange for their own skystead).