We construct our houses aboveground and our skyscrapers into the air for a reason. Mostly it's because we were designed (or evolved) to thrive in the topside environment. We gain physical and mental health from the sun, air and flora and fauna we share the aboveground world with. Should a massive shift to subterranean dwelling take place among humanity, we would essentially be challenging evolution to do its worst. And evolution's worst generally results in costly damages to a species -- like extinction, say.
Humans are diurnal creatures, meant to be active when the sun's up and asleep at night when it goes down. We actually have a circadian rhythm, a biological clock that dictates our slumber patterns based on the cycle of the sunrise and sunset. We're inextricably dependent on the sun.
That link becomes clear through vitamin D. This essential ingredient for human physiological function prevents rickets (improper and weak development of bones) in children and bone loss in the aged. It's also been linked to metabolic and immune system function and reduction in hypertension. So we need vitamin D , and the curious part is that it's the only vitamin humans require that we don't derive from other sources like food or Vitaminwater. We actually produce vitamin D within our cells through photosynthesis, a process that's impossible without ultraviolet radiation (UVR) accepted from the sun through our skin.
We also produce serotonin through sunlight. This hormone is largely responsible for our positive moods, and people who don't produce enough due to a lack of exposure to sunlight can become depressed, a condition referred to as seasonal affective disorder (SAD).
Air is another important ingredient for proper functioning of the human body. Above ground, it's found in aces. The human lung has developed to accept the mixture of elements (mostly nitrogen, some oxygen and traces of argon and carbon dioxide) at the atmospheric pressure found around sea level. Dwelling too long tens or hundreds of feet below sea level, like via scuba, can result in the components of air separating from the blood, becoming bubbles. This creates the life-threatening situation called the bends. The same pressure from the force of gravity is found in the Earth's crust just as it is in the oceans. Miners must utilize the same type of decompression staging as they return to the surface, and, upon rescue, trapped miners are hurried off to a decompression chamber to readjust to sea level atmospheric pressure in the same way rescued divers are.
Of course, there's something to be said for adaptation. Without it, evolution wouldn't exist at all, and a move underground would simply accelerate the process. On the other end of the atmospheric pressure spectrum, generations of humans who've lived at high altitudes, like Tibetans and Andeans, have acclimated to the thinner air. They've adapted to derive more of the scarce oxygen from the air into the bloodstream than sea-level denizens. Underground, our fellow mammal, the mole, evolved to produce and circulate a larger amount of blood and oxygen-rich hemoglobin than similarly sized aboveground counterparts.
Humans could arguably thrive below ground using our most favored adaptation tool: technology. Why wait for eons of evolution to take place and risk the whole survival of the fittest aspect when we can simply whip an environment into the one we desire?
That's precisely what some people who've made the move underground have been forced to do. Rather than adaptation, however, it's generally viewed as addressing design challenges.