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.

Our observations

  • Beneath cities lie thousands of miles of water, sewer, gas, telecommunications, electrical infrastructure, as well as subway tracks. Most cities lack adequate underground mapping because different stakeholders withhold details of the underground (often sensitive and confidential), and light and radio waves do not go through dirt like they do air. Therefore, New York is developing “the world’s most complex underground map”. In London, a huge boom in private basements has led to “Project Iceberg”, which will gather subterranean data. DARPA has launched the Subterranean Challenge to discover innovative solutions to map underground terrain, including the urban environment.
  • Xi Jinping’s plan to build a new city in Xiongan will feature an underground city. Upper sections of the underground will be used for storage, entertainment and parking, while lower sections are to carry pipes and transportation, as well as water storage and other infrastructure.
  • The Boring Company, founded by Elon Musk, is building tunnels in Los Angeles, Chicago and Baltimore. The company’s goal is to increase tunneling speed in order to make “tunnel networks” realistic ideas for cities. For instance, the exclusive use of electric vehicles will reduce the size and complexity of tunnels (i.e. no air systems for carbon), therefore significantly reducing costs.
  • Richard Florida notes that “skyscrapers are like neighborhoods or even mini-cities unto themselves, with some housing 50,000 workers or more.” Based on a new paper, he argues that the rate at which rents rise by floor (vertical agglomeration) is higher than the rate which people are willing to pay for being in prime locations in the city (horizontal agglomeration). In Kuwait’s new $86 billion Silk City, one main development is its “vertical villages”.
  • In Chongqing, a project is under construction with several skyscrapers that are connected by a “horizontal skyscraper”. Indeed, many cities, such as Minneapolis, Hong Kong, and Bangkok, have skyway systems that run between buildings. Furthermore, German engineering firm ThyssenKrupp unveiled its new “horizontal-vertical elevator system” last year, dubbed “the biggest development in the elevator industry since the invention of the safety elevator some 165 years ago”.

Connecting the dots

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.

Implications

  • The underground will increasingly gain attention as a location for urban activities besides living. Tunnel networks are gaining traction from the S. to France and China. In the U.K., France, the Netherlands, South Korea, Japan, and Sweden, underground farming has come into existence to feed urban populations. Infrastructure such as data centers are also being built underground. Notably, all of these options are beneficial to all cities becoming denser.
  • Unlocking the sky of the city will raise questions about access, use and control. “Vertical villages” create parallel cities and could lead to the privatization of public space. Underground development is already raising such questions. London is dealing with billionaires building massive basements. Laws differ widely between countries: for instance, in France, people own the underground of their property, but in Australia, property rights extend just 15 meters down.