In the Venus atmosphere, ~50 km above the surface the air pressure and temperature are the same as here on Earth surface, and you could literally go out and walk with a swimsuit. Furthermore, the atmosphere is made of 95% CO2, meaning that air balloon would have a lifting power due to buoyancy, much like helium in Earth's atmosphere. Thus, establishing aerostat habitats and floating cities has been envisioned.
However, since there are several gasses that are lighter than air, and the same idea (of creating floating cities) could apply to Earth.
The idea is to try to do it on Earth.
To begin, I guess one could start manufacturing relatively small spheric balloons filled with methane (CH4) or (which is cheap, and still has a lifting power comparable to the lifting power of air on Venus). To prevent methane from fires, one would add all of those balloons into another (larger) balloon filled with Nitrogen (N2), which is rather inert, and still slightly lighter than air.
Credits: Inyuki of HalfBakery.