As vast as our planet is, we continue to fill it up with people and overtake its lands with sprawling development. One day, it's conceivable that we might possibly run out of space here on Earth in which to live as we have become accustomed. Many ideas have been proposed to solve this future problem, including: ocean colonization; space colonization; rigidly enforced societal birth control like that practiced in China; rigidly enforced societal death control as seen in the movie "Logan's Run" or on the television series Star Trek. All these ideas have merit, though some seem infringing on basic human freedoms, and all have been seriously explored to some extent.
An elegant, but little practiced option, is simply to use what land we have more wisely. Many architects and scientists have given serious thought to solutions.
Frank Lloyd Wright pondered it in "An Organic Architecture" with his Usonian city idea, called Broadacre city. His image was something like divvying up all of America's land equally for each American family, and he goes on to describe transportation, agriculture, and commerce systems that would support this idea. While this is an appealing concept, there are problems with Wright's solution. It doesn't take into account real and rapid population growth that essentially shrinks the amount of dividable land we have to use in this way. He pre-assumes a more rigid type of democracy than that in which we live. Also, he assumes a more levelled societal playing field where all of us, regardless of wealth or lack thereof, have roughly the same amount of home space or business space as everyone else.
A further solution for this problem, though with some difficulties of its own, is that of Paolo Soleri, who coined the term 'arcology'. In "Arcology: The City in the Image of Man", Soleri describes ways of compacting our city structures in three dimensions to combat two-dimensional urban sprawl. While this led to many science fiction interpretations of domed cities, Soleri's ideas aren't just the "human beehive" model popular in sci-fi. They also encompass vast differences in societal thinking regarding some of the same things that Wright touched upon in transportation, agriculture, and commerce. Soleri deepened Wright's ideas of what might specifically need to be done by exploring resource consumption and duplication, land reclamation, elimination of most private transportation in favor of public transport, and greater use of social resources like public libraries.