A floating city could be built inside a helium-filled ice sphere. The city would consist of several buildings located in the bottom of an ice sphere that is filled with helium. The sphere would be filled with helium becuase helium is lighter that air, so the entire sphere would float. The sphere itself would be made of ice because ice is a low-cost material. Since ice is frozen water, making the sphere of ice would reduce costs since water would cost less than any other building material.
The sphere would float high in the Earth's atmosphere, because the air at high altitudes is naturally cold, it is so cold that the ice wall of the sphere would never melt, it would stay permanently frozen. Since the sphere would be filled with helium gas, a plastic sheet would be required above the city to form a barrier between the air and helium above. The air below this barrier is necessary for the residents of the city to breathe, and the helium above this barrier is necessary to keep the entire sphere floating. If helium is too expensive, the upper part of the sphere may be filled with hydrogen, which is also a lighter-than-air gas.
To build the sphere, a helium ballon would be allowed to ascend to a high altitude, and the helium balloon would release a small amount of water. The water would instantly freeze, because the air at this altitude is always cold. The water would be shaped into a sphere, as it freezes, and this hollow sphere of ice would be filled with helium, and allowed to float near the helium ballon. Shaping the water into a spherical shape would require a mold made of inflatable plastic. The helium ballon would be used as a construction platform. After the hollow sphere of ice is constructed, filled with helium, and allowed to float, the next phase of construction begins.
The second phase of construction involves enlarging the ice sphere. The hollow ice sphere must be enlarged, so it will eventually become big enough to hold an entire city. This process of enlargment is done in several stages. A large shell of ice is built around the ice sphere, and, so, at this stage of construction, there is a large sphere, with a smaller sphere inside it. Both of the spheres are filled with helium. The small sphere inside the large sphere is heated, so it will melt and turn into water. This water is allowed to drain out. After the inner sphere has been completely melted, only the larger outer sphere remains. This process is repeated several times, until larger and larger spheres are built. Shuttle ballons bring additional supplies of helium to the sphere as the sphere is made larger and larger. The ice is reinforced with carbon fibers, to add strength to the ice sphere, and to reduce the possibility that the ice will crack.
Finally, after the sphere has been enlarged to the point where it is large enough to hold a city, the construction of buildings begins. The buildings inside the sphere will be made of lightweight materials, like cardboard and plastic. Since there is no wind inside the sphere, the buildings will never be subjected to wind loads, so the buildings do not need to made of robust materials. The primary purpose of the buildings is to keep in heat, since the interior of the buildings will be heated, and also to keep the residents of the city isolated from helium gas. Ideally, the buildings would be filled with air, so the people that live in them can breathe air, and the atmosphere outside the buildings will be pure helium, this would be the cheapest form of construction. It may also be possible to build a layer of plastic above the city, so the city will be competely isolated from the helium above it. Various systems of helium isolation would be implimented as the sphere is made larger and larger. The most rudimentary form of helium isolation would be plastic tents, for workers. These plastic tents would be filled with air, so workers who were building the floating city would have air to breathe. After the floating city becomes larger, more elaborate and reliable means of isolating the residents from helium gas will be implimented.
Since the air in the stratosphere is always cold, the ice sphere will never melt, and since helium never loses it's bouyancy, the ice sphere should float in the air forever. Why build a city in a hollow ice sphere that's filled with helium? People may build it to escape from environmental hazards on the surface of the Earth. They may build it as a place to take refuge from radiation released from atomic bombs that are detonated during a nuclear war, or they may use it as a place to take refuge from a naturla disaster, such as a volcanic explosion, or an earthquake.
Above: An illustration of a floating city. To see additional illustrations of floating cities, click here.