Land is becoming scarce as the world's population grows and environmental changes shrink the amount of livable space on Earth. Some creative thinkers say the solution is to build up.

A nonprofit organization called Vertical City aims to garner support for so-called vertical cities — Tetris-like arrangements of interconnected towers designed to support thousands of residents.
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These proposed structures, which can be up to 400 floors, contain all the components of a city, from housing and hospitals to universities and municipal departments. Advocates claim vertical cities will save energy, support a growing population, and preserve land for food production, nature, and recreation — if we can figure out a way to build them.