In just a few years, Mexico City will be the most crowded city in North America, perhaps in the Western Hemisphere. With 22 million people, it’s one of the fastest growing populations in the world. There are few tall buildings. Zoning restrictions abound. And it is quickly running out of land on which to expand. Geographic boundaries block it in. This is a metropolis bordered by mountains and volcanoes — the Sierra Nevadas to the east; and lower sierras and ranges to the north, west, and south. For millennia, hordes of people have been drawn to this area because of its wealth of natural resources — pine forests and rivers, wildlife, and even saltwater lakes. There is food and water aplenty. But now there is no more room to grow.

Tokyo, with 38 million residents, is the most populated metro area in the world. It has grown upward, with skyscrapers, and outward without the impediments of hard natural boundaries. Similarly, New York City has jumped the East River and morphed into Brooklyn and Queens and beyond. For Mexico City, there is no such luck. And Mexico City is on track to add millions more people by mid-century. Goldman Sachs says it may become the world’s fifth-largest economy by then. Still, historic preservationists ban renovations. Zoning laws won’t allow developers to build anything taller than five stories in the city’s core. Transportation infrastructure is aged, and the surrounding mountains inhibit connections to satellite towns that could serve as commuter alternatives. The result is mass urban concentration.

Walk through any megacity these days and the crowds are sometimes difficult to fathom. Which is why standing alone in the middle of Mexico City, in the center of its main square — Plaza de la Constitución, or the Zócalo, as it is commonly called — is so surreal. The plaza stretches nearly 800 feet in each direction and is cordoned by national monuments. Yet, there are only a few dozen people at a time crossing it. It is a vast, open area, just a lone flagpole rising high in its center. Mere steps away, outside of this main square, the city is packed, jam-packed with people and vehicles and public transportation congestion.

With as many as 10 million more people poised to be added this century to the already overstressed urban matrix, and with space limitations, the question is, where will everyone go? In all likelihood, down, underground. And they won’t be alone. All around the world, people live underground, entire cities even.

In Coober Pedy, Australia, it’s so hot that people who moved there 100 years ago to prospect for opal realized they had to build their city beneath the surface of the Earth, where it’s markedly cooler. Now close to 2,000 people live there. Coober Pedy’s average annual temperature eclipses 85 degrees Fahrenheit and during summer remains above 90 degrees and often more than 100 degrees for weeks on end. Underground, at depths of more than 30 feet, temperatures remain relatively constant and cooler. Fun fact: The average local year-round surface temperature is what you will find at these depths, no matter where you are on Earth. In Coober Pedy, that can mean a swing of 20 degrees — a big cool-off.

In primitive times, caves did the trick for providing cooler environments and protection from the elements. In modern clusters of civilization, living underground seems like an extraordinary step. That is until you consider this: Global temperatures could spike so much over the next 100 years that it will be nearly impossible for billions of people to remain living aboveground; half of the land area of the planet may become uninhabitable.

A hothouse scenario such as this would mean temperatures rising about threefold more than expected — a lot, for sure, but not outside the limit of possibility. According to a study by researchers at Purdue University and the University of New South Wales in Australia, regions too hot to survive if temperatures rise threefold would include most of the East Coast of the United States, all of India, most areas in Australia, and heavily populated parts of China.

With heat and population concentration in mind (people in affected areas would in all likelihood move to cooler geography, congesting those regions), one solution may be underground living. Besides Coober Pedy, there are major subterranean developments already in the works. In Fukuoka, Japan, architects from the Taisei Corporation of Tokyo have plans for Alice Cities — airy underground spaces connected by subway trains and subterranean roads. In Helsinki, Finland, there is a “shadow city” with a public swimming pool, shopping areas, a church, hockey arena, and an industrial center. In China, old war bunkers have been taken over by people in Beijing. In Singapore, there are plans for an underground “science city” where more than 4,000 people will live. Toronto already has its PATH system: pedestrian walkways that span for nearly 20 miles and connect transportation hubs to restaurants, shops, and other commercial spots. New York is even considering a Lowline park: It is billed as the world’s first underground park. It will use new solar technology to transform a trolley terminal into green space. Breakthroughs in lighting, mood, and spatial aesthetics provide the world with these new underground layers of living and working possibilities.

Visionary architects are teaming with urban planners to come up with living solutions to looming climate change conditions. They’ve partnered before.

In the past, far-out scenarios prompted considerations for building an alternative world. During the late 1960s and into the 1970s, the Italian architectural collective Superstudio crafted designs for a world scant of resources. The group’s architects made blueprints for movable cities — a Continuous Production Conveyor Belt City — among other plans deemed problem-solving. These would be entire communities, or “cities,” that live aboard stations that move about to consume as many natural resources as possible. In 2016, Rome’s MAXXI, the National Museum of 21st Century Arts, exhibited Superstudio’s work, including its 12 Ideal Cities and other concepts that are tragically relevant today and will likely be even more so in the future. The exhibit showed drawings, photographs, videos, and design objects — all of which are fundamentally a critique of society, whether alerting to dangers of overconsumption or warning of the fallibilities of modern infrastructure.

Superstudio architects sought plans for a new world existence. Designs were meant as parables for a world they saw as increasingly headed for the brink of destruction. Today, architects are practically rather than theoretically designing for a world where pollution and temperatures constrict movement aboveground. And they are designing against sprawl.

The most populated cities in the world as of 2019 were Tokyo, as previously mentioned; Delhi; Shanghai; São Paulo; and Mexico City. By the end of this century, the most crowded city in the world will have nearly three times as many people living in its metropolitan boundaries as present-day Tokyo. Lagos, Nigeria, will house 88 million people (it had 20 million residents as of this writing). Kinshasa, Democratic Republic of Congo, will be the second most populated city on Earth, with 83 million — a massive increase from the 11 million people who live there today. Dar es Salaam, Tanzania, will see an even more dramatic rise: 74 million people, up from 4.5 million residents. Mumbai will have 67 million people, three times as many as who live there today; and Delhi’s population is expected to more than double to 57 million.

In past times, without technology such as artificial cooling or heating to solve people’s climate adaptation problems, resiliency was had by re-engineering natural structures as human habitats.

Petra, the ancient city in Jordan made famous by Indiana Jones and the Last Crusade, was once a bustling center of commerce. Its petrified (hence the name) structures were carved into mountains, and an estimated 20,000 people lived there in the fifth century B.C. Major underground cities also existed in China, Turkey, Poland, Italy, and Africa.

In ancient Mayan culture, cities were built on top of one another. The Aztecs built their temples on top of a lake, and then after the Spanish conquest, the Spanish built their temples on top of the temples of the Aztecs. And eventually the whole Spanish colonial city was built on top of the Aztec city. That city is now called Mexico City.

The Zócalo here is either a massive open area meant for congregating and celebrations, or it’s a giant waste of space, depending on how you look at it. Mexican architect Esteban Suárez looked at it differently. Why not, he thought, take a cue from the past, the once Aztec capital Tenochtitlán, and build on top of the cities that form the foundation for Mexico City itself? Why not build down, not up? He designed the Earthscraper, the inverse of a skyscraper, that would plunge nearly 1,000 feet below the surface, jettisoning past ruins and relics, and reaching back in time, metaphorically speaking anyway.

“We thought it would be very interesting, instead of going up with a skyscraper, what would happen if we dug down through these layers of cities?” said Suárez.

But preservationists and city officials quashed it — not before the design got picked up in the media, however. The Earthscraper became a global sensation. It was a finalist in the prestigious eVolo magazine’s annual skyscraper competition in 2010. And people from all over the world contacted Suárez to incorporate the design for their municipal plans. Variations of it were constructed. One riff on the underground concept was even built on the edge of Mexico City itself. Garden Santa Fe is a seven-story-deep underground shopping mall.

Subterranean building isn’t easy, which is why it isn’t done very often. Plumbing has to work against gravity. Foundations have the added weight of earth to work against. Artificial lighting has to be installed to replace areas where natural light would traditionally do. Keeping spaces open, airy, and bright is tricky, not to mention expensive. All told, underground construction can be as much as five times more costly than traditional aboveground building. But Suárez says it’s a must. “We need to go vertical in this city because urban sprawl cannot continue growing,” he says. Satellite towns around Mexico City have been swallowed up in what Suárez calls the “blob” of urban sprawl. The Earthscraper was a solution to try to “verticalize” in an inverse way. “This was an effort from an urban point of view to try to bring new life into the historic center and solve the problem of new living spaces, new commercial and office spaces, that you practically don’t have anymore,” he says.

Suárez’s aim is to incorporate nature into unexpected urban environments. There is a bridge, he mentions, that allows room for vegetation; a pavilion in the shape of a cactus. His designs tell a story. The Earthscraper is an inverted pyramid. Its design has a glass ceiling taking up nearly the entire Zócalo ground area and allowing natural light to filter down through the structure. Green-scaped walkways are lined with natural trees. A museum showcases heritage sights and Mexico’s historic connections to pyramids. (Despite Egypt’s association with pyramid structures, there are more ancient pyramids in Mexico and the Americas than in all the world.)

Different layers of the Earthscraper are devoted to retail, commercial, and residential spaces. Public transportation, as designed, would also pass right through the structure. Made of reinforced glass and steel, the Earthscraper looks, by its architectural drawing, modern, bright, and welcoming. It doesn’t come across as cavernous, which is important. Humans fear being underground.

According to studies, as much as 7% of the world’s population, or about 500 million people, are severely claustrophobic. Indeed, perceived lack of air, light, and exits brings about stress and anxiety in most people. Darkness is our biggest fear. It interrupts sleep patterns and affects people’s moods. Suárez said it harkens to thoughts of being buried alive. Which is why he designed the Earthscraper to capture as much natural light as possible. The large glass ceiling would cascade light through glass floors and walls making its way to the very tip of the pyramid. There, at the bottom of the structure, a water tank would store the rainwater collected from the glass ceilings. There would be tanks for recycling water inside the building, as well as a water treatment plant. The whole place would glimmer bright.

There is something poetic about the Earthscraper rising from this point of water. It would afford its inhabitants the feeling of rising up, back to the surface; as seeds grow.

Not all underground living spaces are as aesthetically minded as Suárez’s design, however. In Beijing, former bomb shelters have been reappropriated for housing. There is no official number of how many people live down there, but some estimates claim as many as 2 million people live below ground.

Annette Kim, director of the Spatial Analysis Lab at the University of Southern California, spent a year in Beijing, observing and researching the lives of “rat people” — the pejorative name given to people who live underground there. She said the conditions vary, from extremely dank and squalid apartments to those not dissimilar to a basement flat in London. Humidity and mold are the biggest health hindrances, she found. “It boils down to design,” she said. Those living underground in dormitory-style settings that are clean and well lit were relatively well adjusted. Those living in places designed as nuclear fallout emergency shelters fared more poorly. One woman told her that living belowground was “dehumanizing.” But China is experimenting with more pleasing designs for the future. The South China Morning Post reported in June 2017 that President Xi Jinping had ambitious plans to develop “a new world underground.” The newspaper said geologists have been examining different underground plots in northern China for commercial use, including shopping and entertainment complexes.

Despite China’s size — the fourth-biggest in the world by land area — people need to live near centers of commerce. That’s why so many choose to live underground in Beijing. They are largely migrant workers. Commuting is expensive. Living underground closer to work is cheaper and more efficient than housing farther away.

The global trend toward more concentrated centers of urban populations may force larger swaths of working-class people to accept subterranean habitats. By night, living with darkness above- and belowground may appear to be the same. For many, though, come sunrise, there may be a different type of commute, a vertical one.

Meanwhile, there is a trend among the world’s wealthy to have doomsday bunkers. These are residences designed to withstand the worst of manmade and natural disasters. They are stocked with supplies that can last for months or even years. They are tricked out with the latest gadgetry, some with swimming pools and screening rooms.

In San Diego, California, entrepreneur and former time-share and real estate developer Robert Vicino has made a community of these high-end shelters, called Vivos. One such community in South Dakota is comprised of 575 bunkers with space for 10,000 people. It is being billed as the largest survival community on Earth. And this is only one of them. Vivos Europa 1 is a 228,000-square-foot complex carved out of solid bedrock, under a 400-foot-tall mountain in Rothenstein, Germany.

Come to Earth in a century and walk along the surface of major cities in the Middle East or equatorial areas, and things might look barren, abandoned. Beneath, though, civilizations might thrive; Earthscrapers underfoot.

USC’s Kim emphasizes that design is key to the health, safety, and psychological well-being of people living underground. By matching new design standards with cognitive ones, better subsurface habitats can be developed.

At the base of the flagpole in the middle of the Zócalo in Mexico City are four transparent square tiles. They cover the lights that shoot up and illuminate the giant Mexican flag that flies above. But there is a fifth tile. It’s made of cement and it is padlocked. It’s hard not to think that what lies beneath may be a solution to overpopulation and urban living.