No Stupid Questions

Not really, no. This is the same kind of silver-bullet thinking as self-driving cars, it may feel cool but in reality the best way to improve things are boring and have been known for centuries if not millennia.
Some things absolutely benefit from being underground, like railways in dense urban areas, but for most things it's just a ton of effort for not much benefit and introducing a bunch of problems (flooding is only going to become more common in the future).

What we should be doing is returning to everything being designed for the specific local environment, stop building everything identically all over the world.
Look at traditional construction and you'll find tons of small features that together make a HUGE difference, a prime example is how hot places had walled backyard gardens with a fountain in the middle, which basically turns the garden into a swamp cooler.

A lot of places already have underground malls, particularly those connected to the subway system. Many roads go underground too, especially when there's a large mountain, ocean, lake, etc. in the way.

But yes, most places are still above ground, as for one, it isn't cost effective to dig out all of the dirt required to create the large underground chambers, the supports needed to prevent collapse, etc. Many tunnels take years or even decades to become finished, an entire city of ground-dwelling people would take ages to dig out!

Aside from being expensive (in both money and time), there's a few practical reasons why this isn't the best idea. You wouldn't get sunlight, so you would need to power artificial lighting fixtures for all the different parts of the area. You could probably save power by creating artificial nights, but still, it would be a monumental task to find enough energy to sustain something like this. Solar panels become out of the question for the most part*, and depending on where you are, you would need good A/C as well, since it can get hot when you're underground, and that consumes additional power. Hydropower could be used, but with underground sources of water rather than rivers. Geothermal energy might become more mainstream as well, which would be kind of cool!

You pointed another big problem, flooding. This would be a large risk, particularly if you live in an area with lots of aquifers (sources of water that are underground) or with lots of permeable soil and rock (so rain easily seeps through the ground), flooding will be a significant issue to tackle. Earthquakes might exacerbate the issue as well, and the shaking of the quakes could also be a problem for the supports preventing the chambers and tunnels from collapsing, like how foundations of our aboveground buildings need to be strengthened. Maybe some sort of moving dampener could be used, like those in tall skyscrapers?

*Theoretically, you could have somebody on the surface maintain the solar panels the cover the Earth, but then you need to build all the infrastructure for somebody to move and live on the surface, in which case you might as well not have everybody live underground. Maybe that could be automated with robots, but that's not feasible with current technology.

Morlocks

Everything you think would be good about underground would be more easily and cheaply accomplished by building aboveground buildings that connect. (Or said another way, by effectively raising ground level to roof level without the expense of digging.)

Underground Atlanta is like this, BTW: they didn't dig below original ground level; they raised the street grid up on viaducts.

You are not a fan of nature and sunlight, are you?

Also radon

I actually quite like the idea of an underground house - it'd be easier to get a nice stable temperature year round and would be much easier to design for bushfire resilience (steel sheets over skylights/doorway, steel mesh over ventilation and you're pretty set). I could do with my own personal hobbit hole.

I would however want it to be in a position where it does not require pumped drainage. That restricts you to places on a decent slope so you can use gravity to keep things dry and cozy rather than rely on pumps that could break down. Not having a view in the vast majority of locations would also be a downside, but to be fair being able to see a good bit of distance is sadly not something you get in most normal houses anyway.

Easier to build the house and cover it with soil and vegetation instead of digging down. The front door and windows can face south to take advantage of passive solar heating in the winter.

I think one of the best uses of "underground" is to run piping in a large circuit around the property. I read in a passive solar book that 4 feet underground it's about 4C on average world-wide.

Summer: Warm air goes in from the living space, travels along a couple hundred feet of pipe, cool air comes back into the house. I'm not sure if the air would be 4C but should save a lot of electricity.

Winter: Same system but the air pumped into the house should be much warmer than what's above ground. Usually, the coldest days are sunny so passive solar designs can warm a house to a comfortable level.

There are several underground spaces where people work, and live. Chicago, Toronto both have underground systems. There is a town in Australia that half the people live underground because it's so hot.

Some issues with underground spaces; it can be expensive to dig the proper tunnels, you have to make sure the geological make up of the area will support the structure, water draining down from above after rain storms can cause issues, and the big one is ventilation, you have to be able to move air through out the entire system.

Do you want Morlocks and Eloi? Because that's how you get Morlocks and Eloi

Underground is not necessarily better for the environment. I think if we compared the ideal underground build to the ideal above ground build, underground would actually be worse for the environment.

Think about it this way: The advantages you might get from underground are related to reclaiming ground and comprehensive city planning. But you can reclaim roof space to make up for the ground, and you can get the same benefits from city planning building above ground.

The idea that you'd just leave pure wilderness on the ground level when you build underground is not realistic. You could grow the crops you need right there above it, instead, for example. A certain amount of land is needed to support each person. But either way, people would be going to the surface every day. If you build underground, you'll also be building above ground.

Meanwhile, underground requires quite a bit more stuff. You have to plan more to manage heat and ventilation. It's difficult to increase density underground because you can build higher more easily than you can dig deeper.

It's much more expensive to build underground.

If we all lived underground, there would be no sound! Have you not heard Jamiroquai?

The tradeoff for resilience in emergencies is that the if the ventilation fails hard enough, everyone who can't get out suffocates. Flooding, as you guessed, is also a serious probelm. Everything that can flood a basement now floods a living space.

Heat and AC are normally going to cost less, but the cost of construction, maintenance, and modification of the structure are way higher and generally kinda dangerous. Not worth it.

It works, when designed well enough. The problem is how do you continue to support the above ground structures, if you aren't as lucky as Chongqing geographically. You have to essentially plan not only all the weight of the structures of each underground level, but also above ground level. That takes lots of highly specialized engineering teams to figure out, which is a huge upfront investment.

Which brings another major problem: Cost. Creating underground structures requires massive mining rigs and blasting and getting rid of the material that comes out and constant review of any potential damage that does when further expanding the city out; all of that costs just so much money compared to normal buildings. If your goal is for-profit development, you'll never break even.

And of course the most obvious problem, most humans are not mole people and do not want to be underground. Sun-deprivation and outdoors-deprivation have serious mental and physical health issues attached, which are not solved by artificial UV-producing lights or indoor plants.

Scarcity of livable land isn’t exactly an issue at this point in time to heavily warrant it.

There are indeed places where large amounts of human activity takes place underground, often being metro systems and their associated retail spaces; Tokyo Station in Japan comes to mind as having an underground mall attached to it.

But the same caveats for underground construction of transportation systems also apply to all other underground structures that humans would like to build. Consider the differences between ground conditions in: the San Francisco Bay Area, Denver, and New York City.

The Bay Area is the outlet for major rivers in northern California, bounded by mountain ranges on virtually all sides. The surface is either a thin covering of soil atop this mountain rock, or is a layer of looser soil or mud, made from the sediments carried in by those rivers. This makes for fantastic agricultural conditions but presents a real risk of liquifaction when there's an earthquake. While an underground structure wouldn't fall over -- because it's within the ground -- it could certainly lose its supports unless it has piles all the way down to the rock. And that's only buildable on the narrow shoreline region where there's sufficient depth before hitting the rock layer.

With Denver, it's basically all rock, so to build within the rock would require blasting it away and building within the hole, or to build normally then bury the structure in fill, so that it's below grade.

With NYC, it's a different story because the ground conditions make it fairly easy to dig tunnels and drive piles, and the bedrock layer beneath Manhattan is strong enough to support the weight of supertall-class skyscrapers. On this point, the New York Fed's Gold Vault is in the basement in Manhattan, precisely because the volume of gold inside would be a serious strain on any foundation and the geology beneath.

All that said, the surface conditions in some extreme climates may warrant building underground, or avoiding the underground outright. Burying a dwelling in New Mexico would make a lot of sense, due to the hot and dry Southwestern climate. But in Alaska, an underground dwelling would cause melting of the permafrost layer below, resulting in a similar situation to liquefaction. I suppose this can be mitigated, but it would be a monumental effort, akin to Camp Century in Greenland. That project was abandoned due to changing ice geology.

I've always wanted this. Underground cities would be so dope but everyone wants a 40x40 of domestic grass to do nothing with.

https://en.wikipedia.org/wiki/Derinkuyu_underground_city

I think it would be great for a lot of our transportation to be underground. Underground roadways if needed, otherwise a much expanded subway system in cities

Road cause so many issues it's ridiculous. Have all the roadways and motor vehicles underground and only pedestrian/bikes up top with green spaces and public areas.

Singapore? Or what country are you talking about?

Better? Probably not. Maybe it eventually starts making more sense with extreme temperatures.

I would be interested, but looking at the cost of something as remote as Whittier, AK does not inspire confidence. Though even if it were non-monetary, I still probably couldn't pull my own weight. (also getting there, social compatibility etc)

we should put coober pedy underground

Well you see, we need windows so we can see all the grey and tan boxes outside. How could we live without the starless night sky? Or the ten minutes of sun through your window by the time you get home from work?

But in all seriousness, You need jobs first and foremost because otherwise everyone would just rather live rurally. You wouldn't want it to become a company town nor ghost city, so there would have to be a heavy investment by dense congregation of successful businesses. You'd basically have to decide how deep you're going to go, then tunnel massive amounts of sewer type systems beneath that. The biggest thing imo is dealing with heavy gasses. That is a lot of expensive infrastructure to prepare very deep, before anyone can even start thinking about living spaces. Beyond that, I don't think the challenges outweigh the challenges we face now and in the near future. We've adapted to those, we could adapt to more, but it'd be a lot of expensive learning experiences in the mean time.

It might help to frame it as living under a park, or living within walking distance of businesses that necessitate or benefit directly from being outside. Like cheap food from small farms maybe. Termites massively out-number humans by mass, but they seem to make a pretty good go of it without devastating their ecologies.