Climate-controlled domed cities are another potential solution if the external atmosphere becomes too hostile for habitation. A sealed bio-dome would provide a habitat with controlled air temperature, air composition, and air quality. Such a regulated ecosystem would allow citizens to continue their lives undisturbed by whatever conditions were outside the dome, such as extreme temperatures or pollution. Proponents tout these cities as more pedestrian-friendly, cleaner, and more secure. Amazon’s Spheres serve as a small-scale example. Saudi Arabia unveiled designs for its ambitious urban project The Line, a mirrored one-building city in the desert that will stretch over 106 miles and potentially house 9 million residents. Part of the $500 billion Neom project, the proposed futuristic city will be the first to be totally powered by renewable energy, including wind, solar, and hydrogen. This will ensure a zero-emission, carbon-positive ecosystem. The city is built around a concept of “zero-gravity urbanism,” whose objective is for citizens to be able to satisfy all of their daily needs within a 5 minute walk. Parks, grocery stores, transportation options, and more are layered on different floors of one enormous structure, enabling a much denser use of space.