Believe it or not, a floating city might be a feasible project. Scientist and science fiction author Geoffrey Landis presented a paper called "Colonizing Venus" [PDF] at the Conference on Human Space Exploration, Space Technology & Applications International Forum in Albuquerque, New Mexico back in 2003. Breathable air floats in Venus's soupy carbon dioxide atmosphere, which means on Venus, a blimp could use air as its lifting gas, the way terrestrial blimps use helium to float in our much thinner atmosphere. ...
One of the biggest problems with a lunar or Martian colony is that an astronaut’s bones and muscles deteriorate in low gravity. No one knows yet how much gravity a human needs to prevent deterioration, but Venus's gravity is the closest to Earth's, at about 9/10ths. Mars only has a third of the gravity that the Earth does, while the moon has a mere sixth.
Atmospheric pressure is also crucial. Think of the difference between jabbing a car tire and letting air out of a half-inflated balloon. Gases seek equilibrium. Since there's barely any atmosphere on the moon or Mars, a rip in the hull of an enclosed human habitat would suck oxygen out at tremendous force. Thirty miles above Venus, it would merely seep out. This also means a Venutian cloud colony wouldn't need as much reinforcement. Venus has other boons, too. Its rich atmosphere blocks radioactivity and could be mined for useful materials. And with a gentle temperature, far less energy would have to be spent on heating or cooling the colony.
--James McGirk, Citylab, on a new planet to love. HT: Chris Blattman