Maps could fool us into thinking that cities only grow horizontally. Naturally, there is also the vertical city: from the underground to the sky. As urbanization rapidly accelerates, the vertical city is gaining momentum. Ambitious plans for tunnel networks and bridges between skyscrapers have emerged. The underground and the sky, both diametrically opposed and intimately connected, will transform urbanization.
In the early 20th century, science fiction imagined cities in which people could live underground and fly around in cars. In fact, the reverse has happened: we live in skyscrapers, while using underground transport. Nonetheless, the vertical city reveals its rising relevance in the development of cities. Indeed, these earlier ideas about the vertical city could still come to fruition. Above all, while the underground and the sky are wildly different, they both offer solutions to challenges that cities face on the surface.The sky of the city is prestigious. Just as the upper world of the sky is occupied by the gods in many religions, penthouses at the top of skyscrapers are widely admired by passers-by. High-rises offer better views and less noise. However, for cities, high-rise buildings are increasingly more a necessity than luxury. The global housing crisis drives cities to build more affordable housing, which is made more feasible by building high and densely (as opposed to low and sprawlingly). Furthermore, the vertical city becomes more important due to horizontal limits (e.g. lack of space, zoning regulations, travel-time budget). Therefore, obstacles to building underground will naturally push cities to reach higher. For instance, dangerous uncertainty surrounding subterranean construction, the high costs of digging, and negative perceptions about being underground could limit meaningful development. Consequently, cities are forced to enhance efforts to build higher, instead of deeper.The underground is something we would rather avoid. Just as the underworld is the world of the dead in religious traditions, few people today would want to live there. After all, the underground is where we hide things, such as sewers, cables and pipelines. It is also largely unmapped territory. Nevertheless, rapid urbanization could increasingly force cities to expand underground. After all, high- rises mostly enable living and working, but the underground could potentially contain much more of the city. Indeed, the most important drivers of building underground are related to the limits of high-rise construction (e.g. zoning regulations for height, soft soil, the fueling of congestion by building too densely). Furthermore, in places such as Toronto (its PATH system is one of the biggest underground systems in the world), Singapore and Hong Kong, the climate stimulates underground construction.The vertical city will increasingly shape the future of cities. Indeed, cities will expand both into the sky and underground. The steady rise of the average height of buildings since the mid-20th century is likely to continue. When proper connections emerge between skyscrapers, these vertical villages could create new urban communities. Meanwhile, however, such high-rise development will worsen congestion on the ground. The underground will therefore increasingly become an attractive alternative to depressurize the city. Indeed, besides living, other types of activities could increasingly move underground (e.g. transport, food production, digital infrastructure). All in all, while the symbols of the underground and the sky reveal their diametrical opposition, they also show that the vertical city is tightly connected: what happens in the sky and on the surface will influence the underground, and vice versa. Hence, to anticipate changes in cities requires thinking of the city as an organic system. For instance, self-driving cars could reduce the need for high-rise construction, horizontal-vertical elevators could reduce congestion, and climate change could accelerate underground development.