Underground Cities will be a reality in the human future, for multiple reasons. Many of us, especially claustrophobes, view this prediction with horror and disgust, but that’s unnecessary. Subterranean existence has been wildly successful for many species like earthworms, gophers, ants; homo sapiens might be the next ecstatic burrowers.
Who’s happier than a meerkat, really? If we dig deeply enough into the soil, perhaps we can achieve the giddy socialization of the chatty Kalahari mobs that are feminist and cooperative models of altruism.
Why would humanity descend into the bowels of the earth? Here’s four catastrophic reasons:
- Escaping Nuclear Bombs, with their explosive force of 2,000 mph, millions of heat degrees, and deadly radiation. Moscow already has enough bunker facilities for its entire population, plus the military and government.  Switzerland has enough bunkers to hide its entire 8 million population, and Sweden has more than 65,000 shelters, enough for 70% of its population.
- Avoiding Pandemics. Hiding below, away from plague-winds on the world’s surface, is an excellent way to escape global diseases instigated either naturally or via biological terrorism.
- Staying Cool with Global Warming. Experts predict many major cities will be uninhabitable by 2100, roasted by fatal heat waves, notably in India, Pakistan, and Bangladesh. 
- Surviving Meteor & Asteroid Attack. Dinosaurs were wiped out 66 million years ago by a space rock.  The impact tossed dust into the atmosphere, blocking sunlight; this eventually killed 75% of life on the planet.
Additionally, here’s three reasons that are less urgent, with significant benefits:
- Underground cities can populate inhospitable areas that are a source of potential wealth; Greenland is an example — due to future mining for rare earth minerals. A “nuclear winter” could also impel humanity to hibernate underground, like the snow fox.
- Activists believe planting a trillion trees can help alleviate the chaos caused by climate change. Contributors to this effort might choose to get-out-the-way by dwelling under the extensive forest.
- Many mega-cities are over-crowded but unable to expand horizontally. Singapore, Beijing, New York, and Mexico City are already digging into their ground to provide more residential, office, and recreational space. 
Living in Underground Cities can also protect inhabitants from wildfires, tsunamis, conventional warfare, toxic air, volcanic activity.
Truth is, humanity has always built communities in the planet’s interior. A 5,000 year old underground city was recently unearthed in Cappadocia , large enough to accommodate 20,000 people, plus their livestock and food, with tunnels seven kilometers long, and hundreds of escape passages. Similar constructions took place in Naours, France and Orvieto, Italy around 2,000 years ago.
Today, Montreal’s RESO, aka Underground City  contains 20 miles of tunnels and 35% of the city’s commercial area, with 1,000 offices, 2,000 stores, nine hotels, 200 restaurants, four universities, 40 cinemas. It is visited by 500,000 people daily, who enjoy the warmth in winter. Helsinki, in Finland, also has extensive underground amenities.
Three negative aspects of underground living are:
- Losing the view might mean losing a bit of brain resilience and happiness. Studies indicate views of nature, especially trees and mountains, provide healing “restorative power”  that accelerates surgery recovery, decreases pain, and elevates happiness and attitude.
- Claustrophobic fear of enclosed spaces afflicts 2–10% of the population.  Claustrophobes would demand to live near exits, plus they are prime candidates for hygge — the Scandinavian term denoting joy of winter hibernation with cozy social togetherness. 
- People deprived of sunlight can suffer from insomnia and hormone dysfunctions. This danger can be alleviated with supplements and artificial light duplicating the properties of solar rays.
Underground cities would probably take the shape of inverted skyscrapers if society was structured with a Georgist economy that levied a single, land value tax on the size of the surface ground. Georgist economies would encourage underground building, because “stacking” chambers underneath each other would result in a lower LVT cost. Georgism supports vertical development, up and down. 
Today’s present technology is fully equipped to build underground. Oil drills plunge 6,000 feet down; tunnel boring machines horizontally excavate 60 feet per day. Montreal developed air purifiers that not only extract dust, allergens and odors, they also destroy 99% of the Covid-19 virus. Plant food can be easily grown hydroponically underground with grow lights; mushrooms actually grow faster in the dark, or with minimal light.
Is dwelling underground what we want? For most of us, no, but it is reassuring to realize the option exists. Ideally, if it happens, our psychologies will adjust, and perhaps even thrive, like the comfortable meerkats that snuggle in heaps, groom each other, and cheerfully share all responsibilities.