Between the beginning of the 19th Century and the beginning of the 21th Century the population of the world grew from approximately a little over a millionto about six and a half million. Cities, which until then had been essentially horizontal, added a second tier, mostly in the West, but also elsewhere in the world, growing vertically skyward, with the addition of high-rise buildings. Cities with populations of ten million or more will number about sixty within the next seven years. Cities will have to expand vertically again, downward, this time to add a third tier.

For the cities to continue to grow only horizontally to accommodate this explosive growth has meant the expansion of vast urban slums surrounded by suburban growth, putting unsustainable pressure on extended roads, streets, sewers, water, power, communication and energy systems. Continued vertical expansion skyward with densely packed skyscrapers only adds to the increasing congestion on the surface. The parks, plazas and great public open spaces through which cities have not only offered relief from crowding, but conducted their most important public affairs, bringing people together to maintain a civic and peaceful society, will become increasingly relatively fewer, lost in a warren of mean streets and alleyways.

There’s nowhere else to go but down to add usable building space and increase surface open space at the same time. This will require a bold re-thinking of how we define urban space, how we design our buildings, and how we integrate them with the array of utilities, services and transit already in place.

In urban areas, many of the spaces designed for commercial and public activities are essentially interior—or a combination of largely interior function with some need for an exterior public exposure. In this category are theaters, museums, stadiums, shopping centers and transit centers. For significant of parts of each of these types of uses, exterior exposure is not only unnecessary, but undesirable. Although sports arenas can function, and have functioned, as outdoor spaces for thousands of years, more and more enclosed, climate-controlled stadiums are being built –on the surface, enclosed with blank walls. Significant parts of most theaters and museums do not and cannot function as outdoor spaces in their principal areas which are designed for presentation and display. Theaters in particular need to have complete control over the use of light. Modern 20th and 21st century shopping centers, for similar reasons, are almost always designed so the outer shell is completely blank, shielded from daylight, except perhaps from above. Because they are so enclosed, these kinds of spaces require as much mechanical ventilation as they would underground, where the cost of heating, cooling and maintaining them would be substantially reduced.

At the same time, most of these building types have an equally compelling need for visibility and exposure to attract users and to serve ancillary functions for social activities— and often to make a cultural statement through their architecture. Almost invariably, they are enclosed in exterior envelopes which their designers strive to make more meaningful and interesting by the fevered manipulation of geometry, materials, and color. Yet, the Louvre museum addition in Paris, where a single relatively small glass pyramid in a large surface open space with an equally large museum below it, has all of the iconic power of any major building of this type constructed in the last fifty years, and stands as a powerful example of the three tiered approach to urban expansion. There are other examples.

All of this may seem very specialized and peripheral to the concerns of the vast majority of people, particularly the urban poor, who populate the cities around the world. It is not. Apply this approach to every public building, every school, every community facility, every hotel, every housing complex which has an auditorium, gymnasium, theater or other gathering space which is an essentially interior space which could just as well be constructed underground, leaving more space available on the surface. Multiply these by hundreds or thousands in any city of any size, tie them to subway transportation and the shape of the city changes – more compact and with more surface open space at the same time.