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Humans have always needed protection from the elements, but nature provided limited
shelter and, until we could build our own dwellings, the Earth itself provided some
of the best sanctuary. Perhaps it will again in the future too? Living underground or in caves is nothing
new to humanity, as the Flintstones can attest, and today we’re going to be exploring both
how we could build expansive subterranean colonies in the future and why we would want
to. That’s an important first point though:
we aren’t breaking new ground here, but exploring new ways to employ existing technology
and methods. Even ignoring trivial examples like basements
or mine shafts, most large cities have large subterranean networks such as subways, sewers,
piping, and cables, along with their maintenance causeways, and a few even have underground
commercial districts. There’s a lot of reasons why we don’t
typically house people and offices underground. First there’s the cost issue: most buildings
are only at ground level, and where they exceed that it’s usually a second floor, an attic,
and single basement. Those mostly exist because you have to dig
down to make sure your foundation is solid, and you’re usually only digging into dirt
and clay, not bedrock. Cost of excavation is huge, far more than
building up to an equal height is, and when we drive pilings down many meters for tall
buildings so they don’t tip over, it’s an engineering necessity rather than with
the intent of building many subterranean layers. As mentioned, residential basements often
aren’t fully submerged either. They typically poke up above the ground level,
which is often mounded up higher at the house than the rest of the yard. This lets in light and air, two things that
basically required windows until modern times anyway. People like sunlight and fresh air. The health aspect of living in a basement
isn’t as bad these days, as we can condition and filter air to keep it free of moisture
or mold, and remove stuff like carbon monoxide or dioxide or radon, all things that made
basement apartments quite unpleasant in the past. Our ancestors had lamps and candles, which
were very expensive and produced a dim weak light, as well as producing smoke and being
a potential fire hazard. We have light bulbs that can simulate daylight
just fine, and vitamin D supplements are available to replace deficiencies someone can develop
from not getting enough UV, not to mention tanning beds. In a skyscraper you can’t open the windows
for fresh air either, so the only element missing is the view. Those safety issues don’t stop there though,
if a building catches on fire, a truck with a ladder can retrieve victims from higher
levels, and the buildings themselves can have fire escapes they can flee down. That’s a little trickier if you live in
the tenth level basement of a building whose eighth and ninth levels have caught on fire. You’re pretty thoroughly screwed. Unless of course there are tunnels connecting
that floor to other underground buildings, just like we often build skyways between the
higher levels of buildings. As we push toward more three-dimensional cities,
we’ll see today what’s likely to be the main reason we develop subterranean cities:
as extensions of existing ones spreading out wider and higher. One other note on that, building is expensive
when you go vertical, be that up, down, or both, and the cost depends a lot on what you’re
building on top of. Some material is easier to excavate down to
bedrock, while other kinds are unstable enough that you need to go deeper to achieve a solid
foundation. Whether you’re building on sand, dirt, clay,
swamp, or bedrock makes a big difference, but regardless as to what a city is first
constructed upon, over time what it’s mostly built on top of is itself. We get layers of cities, and that’s also
one way you can end up underground; not because you started there, but because everything
around you grew upward. We’ll come back to that, and deal with issues
like a bad view or motives for wanting to live underground in a bit, first let’s talk
about the classic one: external danger. People have often taken refuge in caves or
basements, and almost every film dealing with an impending apocalypse will suggest mankind
could go live underground to save a remnant. The film Deep Impact shows a massive construction
project to build survival shelters against an asteroid strike and lottery for who gets
in. Doctor Strangelove, one of my favorite films,
has the titular character discuss such a plan at the end of the film, moving humanity down
into old mines until the fallout settles. How realistic is this? How many people could you get down into one
shelter and support? And what sort of problems would you face? First we should dispel a basic misconception. You are unlikely to have people crowded in
so tight that they’re sleeping on cots in a grimy hallway. Personal space isn’t really the issue in
a long-term bunker any more than it is in a giant arcology. You need so much more room for storage or
production that personal space is fairly trivial. And they certainly aren’t grimy, we are
not idiots and a dank, dark, over-packed grimy cave is practically an open invitation to
Nurgle, God of Pestilence and Decay. Considering it’s a doomsday bunker, which
usually implies plague is already a serious concern, someone is likely to be tasked with
enforcing sanitation and hygiene. If you’re a survivalist or prepper, you
can have all the canned food and MREs you want, all the medical supplies and guns and
ammo and anti-zombie spray you can find, just don’t forget the Lysol. Second, if it is a genuinely long-term bunker,
someone is trying to grow food in it, even if just to help with rationing out your stockpiles,
so your corridors are more likely to be packed with hydroponic shelves and well lit. Psychology is an important factor when we
consider housing people in the long-term in space, and improving morale with bright corridors
sheathed in plants is a bit of a no-brainer if you’re stashing people underground for
years at a time. Of course you need a lot of power to do something
like that, and it also depends on what the crisis was that drove you underground. Realistically there aren’t many. While things like Nuclear Winters or Volcanic
ash can wreck a planet’s ecology for generations, the period where it’s actually dangerous
to be above ground is fairly short, even if we discount the fact that underground locations
tend to have air and other resource issues themselves. They’re also frequently relatively high
in radiation, even before considering that you are probably using nuclear power to run
one of significant size. A vast underground farm doesn’t have much
going for it that an above ground greenhouse wouldn’t also have, and you’re only growing
food down there if you plan to stay a long time, anything less than a year and you’d
just use stockpiled supplies instead, maybe supplementing them a bit to get fresh veggies
while recycling air and waste as we discussed for long space voyages to other planets. Let’s run through the plausible disaster
scenarios. Asteroid Impact in the ocean, big tsunami? You hide and go out and start planting again
the next season. You probably only retreated to higher ground
too, not underground. Asteroid impact on land? Wait for the dust to settle for enough light
to get through, and build greenhouses to block out wind and dust storms instead of underground
farms. Similar concepts apply to nuclear winters. For the most part, there’s not really that
much radioactive material lying around to begin with, and you can greenhouse an area,
decontaminate it, sequester the radioactive bits, and just keep expanding that. Far easier than drilling out and shoring up
big caverns. Supernovae ionize the atmosphere and wreck
the Ozone layer. Again, greenhouses, utilizing materials that
filter or reflect specific types of light, would work fine. Ditto for a gamma ray burst, and in both cases
if you have advance warning to be building a vault, you probably can get away with building
a shield in space instead that can take the hit for you, something we discussed in our
last collaboration with Joe Scott on various ways the world might end. If you’ve got the power to light underground
farms, you’ve got the power to light ones on the surface. Your one big advantage to the former approach
is that you’ve got temperature on your side. Underground is decently warm, a great insulator,
and indeed can get quite hot the deeper you go. This is a costly advantage though, since drilling
through rock is a lot harder than building a sturdy structure and covering it with a
protective layer of dirt, unless heating the place is more energy intensive than lighting
it, or if for instance your whole atmosphere was blown off or made toxic. Of course you can save the costs of excavation
if you use existing underground features or mines. Some of these features are quite large and
can house large populations. Unfortunately these are often not near large
metropolitan areas and would still require large infrastructure improvements to be habitable. So you might retreat to old mines and bunkers
temporarily with supplies and re-emerge around them with greenhouses or covered buildings,
but you wouldn’t expand that cave complex too much. Though you might do both at the same time,
especially off Earth on places like the Moon or Mars, where excavating is often easier
and the low gravity lets you pile more dirt on structures for shielding. As you dig out more caverns, you get rid of
your spill by piling it on structures you’re erecting around your bunker complex on the
surface. Again one way to become a subterranean culture
is by building up, rather than down. Many of these tactics also work better on
other worlds and might be an example of learning techniques elsewhere that come in handy back
here. For instance, tidally locked worlds, which
are probably decently common in the Universe, have a dark side that’s actually dark, quite
possibly covered in ice like Antarctica, but certainly cold and dark, and building down
deep there is a lot more appealing. The galaxy is also full of various icy moons
and dwarf planets, likely orders of magnitude more of them than Earth-like worlds. It’s much easier to melt through ice than
rock, and the lower gravity would make it much safer and easier to make large tunnels
and caverns. Indeed many such low-gravity worlds will already
have a ton of natural caverns. You can get some very large lava tubes on
small dead worlds with little gravity or erosion to collapse them during formation or afterwards. They also tend to be pockmarked with craters,
and craters within craters, and if you build in one of these and push a layer of protective
regolith over top of it, that’s basically a subterranean city. But the motives to do it here on Earth, especially
without some apocalyptic incentive, seem minimal. So let’s look at some reasons we might do
it here if the world was prosperous and peaceful. First, again we already build underground
in many cities, especially where it gets cold. Subsurface structures save on heating bills
and snowplowing. We also do it with subways, and considering
the biggest logistical nightmare of building freeways and railroads is right of way and
acquiring a straight chunk of land, drilling through rock might turn out to be cheaper
if we get better at doing so. And we are, our drills are better and more
efficient, our construction to brace those tunnels cheaper and better than before. We can make synthetic diamond drill bits and
with improvements in metallurgy for high-heat application, new technology is going to let
us cut tunnels faster and cheaper. You will still cut them though. It’s popular to suggest vaporizing tunnels
with lasers but that’s not really logical, in fact it’s basically crazy. The amount of energy needed to vaporize rock
is orders of magnitude higher than to cut it and drag it out, and a keyword in there
is ‘vaporizing’: you’re turning it into gas, in a confined tunnel, which means it’s
now a giant shotgun barrel. However, this does work with ice, you can
melt caverns much more efficiently, but only because it takes much less energy to melt
ice than to vaporize rock and furthermore because it can be done with waste heat. We don’t live at below freezing, so our
homes are giving off heat in a frosty environment sufficient to cause ice to melt, but obviously
not high enough to melt rock, let alone vaporize it. As we move into subterranean cities rather
than doomsday bunkers or offworld colonies, I just want to clarify terminology. We’ll assume something qualifies as subterranean
if there’s a bunch of stuff above it blocking out sunlight, be that the ground or ice or
the peak of a mountain you’ve tunneled into, or an artificial mountain like a giant mega
metropolis or arcology. For places like that, where you’re just
at the bottom of a city, the most obvious motive for living down there is that it’s
cheaper or more convenient. The roads, for instance, are at or near ground
level and there’s more foot traffic, so it’s a good place for stores or other highly
accessible facilities. The basement is cheaper than the second floor
though, as it’s less desirable, and often less well maintained too. That’s a popular trope with megacities in
science fiction: the vast lower levels of the hive city, populated by the destitute,
criminal gangs, and mutant cannibals. Warhammer 40k uses this one a lot, with huge
hive cities kilometers tall and deep that just build the new right on top of the old. We see that theme on Coruscant, the capital
of the Empire and Republic in Star Wars too, a ground floor so far down from the peaks
of skyscrapers, that no sunlight ever reaches those levels and whole sections have been
abandoned. Such places are scavenger realms, fed like
the ocean depths by the marine snow of detritus that falls from above. Hive Snow, I suppose. Whole regions might be flooded by cisterns
or sewage treatment plants bursting above. Personally I love that imagery, one of the
rare occasions something in more dystopian science fiction doesn’t make me roll my
eyes, but it obviously isn’t a place you’d want to live and it inevitably begs the question
of why the whole place doesn’t fall down? Of course we don’t fall down, but that’s
because we’re on solid rock that’s as compressed as it can reasonably get, although
cities do tend to slowly sink for a variety of reasons. One can imagine a super-strong material being
used in construction too. If a mountain of natural material can exist,
then a mostly hollow structure built by talented engineers of high tech materials can exist,
and be bigger too. It would inevitably be very over-engineered,
and could likely handle many extra layers being added over time. We discussed back in Dying Earth that long-lived
and prosperous societies that are more worried about getting rid of heat than anything else
might be prone to simply sealing up someone’s home when they die, and just keep building
layer after layer. After all, death can be pretty ambiguous in
high-tech civilizations, and people might not be too comfortable moving into someone’s
apartment that they dwelt in and left their mark on for dozens of centuries, but they
might not be sure they were entirely dead either. In a post-scarcity society where the rent
and taxes on a place might not exist or just keep getting paid automatically, and where
robots run around maintaining everything, and where people might routinely live in virtual
worlds or be digitally uploaded or go into stasis for centuries at a time, it might be
rather hard to tell if a place is genuinely abandoned and you might just build a new place
higher up and treat the old as a tomb, possibly a temporary one. It also raises the verticality issue of land
ownership, do you own the sky above your house and how far up, or down? If someone wants to cut a tunnel under your
home several hundred meters below, or connect to adjoining towers by a skyway passing over
your home, can they do that? The law is still rather iffy on that and varies
from place to place a lot, but we’ll have to decide sooner or later. How deep? All the way down to core? How high? All the way up to orbit? Is it a column or a wedge growing skinnier
as you go down and wider as you go up, since the planet is a sphere not a flat plate. That’s an important question because as
I said earlier, one of the biggest problems with new railroads or freeways is getting
right of way to build one in a straight long line. That’s easier if it’s underground. What’s more, we have an option on the table
for rapid movement around the planet that’s a lot more energy efficient than flight, and
that’s vacuum trains. Big hollow tubes emptied of air or mostly
so to minimize drag, allowing ultra-efficient and high speed transport. One often raised issue with those is concerns
about the tunnels expanding or contracting over the course of a day as temperatures changes. I’m not really sure why some folks think
that’s a big issue, since we solved it with railroad tracks and gas and water pipelines
long ago, but while it’s manageable it is certainly a pain. A constant temperature is much easier to build
with, no expansion joints or metal fatigue issues, and temperature stays quite constant
deep under the ground. That’s very handy for such tunnels and for
all manner of other construction that has to deal with large objects under a lot of
pressure, like giant buildings. Of course temperature actually does not stay
constant underground, it just stays constant at a specific depth, as you go lower the temperature
rises, often as much as 30 Celsius per kilometer or about 80 Fahrenheit per mile. That makes it potentially awesome for geothermal,
but rather hot to live in. You could cool that while also harvesting
that energy, but I should also note that depth doesn’t just raise temperature, it raises
pressure too. Same as going up higher lowers air pressure,
like on a mountain, going lower will raise it. Not as fast as being under water will, but
we have mines over a kilometer deep, some much deeper, and pressure can get quite high
down there. We can handle higher pressure decently enough
to a point, and indeed many folks pay good money to spend time in hyperbaric chambers,
but only to a point and you’ll have secondary effects too. A lot of chemistry is fairly sensitive to
pressure and that’s not limited to biochemistry or what temperature water will boil at when
you’re cooking. It can affect chemical reactions like corrosion
or rust too. You’d also likely see adaptation in plants
or animals that lived in such places, not just the obvious ones like dealing with darkness
but unexpected ones like lungs modifying to adjust to different air pressures. Fiction might show us twisted albino morlocks,
mutants, or dark elves flinching from the sunlight, but doesn’t show them gasping
for air on the surface the same way you or I would if climbing a tall mountain. It also doesn’t show the folks fleeing them
in the dark tunnels rapidly outpacing them because they take longer to get winded being
used to the thin air of the accursed sunlit realm. Those would be natural adaptations though,
and we can presumably go for artificial paths. Folks will sometimes suggest skipping the
sunlight issue of underground farming in favor of things like mushrooms, and indeed nature
and science can both produce plants that could live on chemical or geothermal energy, though
they’d never be as fast growing as sunlit ones, just not enough juice. Still, they say the land was once covered
in trees like prototaxites that they think might have been a fungus, and the largest
organisms on the planet even today are fungal mats stretching over whole kilometers. You could probably breed and certainly genetically
engineer huge mushrooms forests to fill vast underground caverns. And I’d say that would be quite a sight
but since you are doing this to avoid lighting the place you presumably can’t see much. Though mushrooms hardly explode when exposed
to dim light, and I could certainly imagine us building underground cities with twilight
lit mushroom groves and parks. Deep underground is a nice place to put factories,
their pollutants can be easier contained, and it’s a great place to use as a warehouse
too. As we’ve often said, colonizing a place
does not necessarily mean a lot of folks live there, so you might have mostly automated
underground factories and warehouses, but someone presumably lives there so some mushroom
gardens might make it easier to gets folks to migrate there or even attract tourism. The mushrooms would be handy too. Besides being tasty we’re finding a lot
of additional uses for them like making fake leather. They’ve even been experimenting with using
them as a construction material, and big ones designed to mimic trees might get cut down
for the equivalent of lumber too. Amusingly they’re also one of the more plausible
pathways to plant-based intelligent life, ignoring that they’re not actually plants
but fungi. We usually reference science fiction rather
than fantasy here, but I mentioned dark elves a bit ago along with morlocks, and if you’re
familiar with the Underdark from Dungeons and Dragons, where those dark elves live,
they also have a race of critters called the myconoids, big walking mushroom people akin
to the Ents of Lord of the Rings. A difference between things that live on chemical
decay, like mushrooms, and a photosynthetic plant, is that the latter wants to root down
to one place and stay there, because the Sun will rise tomorrow, whereas something decaying
eventually stops as it runs out. So one could imagine mobility evolving into
such organism, a basic animal that can get up and find a new pile of rotting wood or
manure to dwell on, developing senses and a nervous system and even a real brain. Not very likely but not impossible either,
and of course genetic engineering opens up many possibilities. I could easily imagine us making fungus-animals
and intentionally boring vast caverns to fill with ecologies tailored to the dark or to
a dim light. Besides, who wouldn’t want to talk to a
giant mushroom person? He’d probably be a fungi. Even if you have the energy to light every
cavern to daylight, you don’t actually want to if you want a lot of space because light
produces heat and you have to get that off the planet somehow. Vertical farming lets us circumvent limits
on land area, be it by going up or down, but you can only do so many layers with sufficient
lighting before things get too hot, so keeping things dim lets you have more space. A huge crystal cavern full of stalactites
that only dimly illuminates when someone enters it is a lot energy cheaper, or heat-cheaper,
than some noon-time illuminated park, and a more vertical future civilization on earth
is likely to embrace the twilight and darkness more with that in mind. Even plants that need bright sunlight can
actually do just as well on a much dimmer and tailored spectrum and since our eyes are
logarithmic in sensitivity, we don’t really think of our living room lighting as being
a thousand times dimmer than our lawns at noon. Nor again is underground limited to the actual
ground, it would be very easy to imagine a spaceport being built at the top of a mountain
and the folks living there hollowing out the peak so they could pressurize it and keep
it warmer. I suspect there you’d go with giant windows
or mirrors built into the slopes rather than a lot of artificial lighting, but you might
do that in genuinely underground places too. Nothing is stopping us from cutting a big
shaft or narrowing pit into the ground that we lived around the edge of and maybe domed
over with glass to help with heating and flooding when it rained. That still leaves us with one big issue, and
that’s earthquakes. Those are devastating on the ground and far
worse underground. Obviously you don’t want to build underground
a lot near fault lines, but those often are the best places for geothermal too and that
along with mining and drilling are two of our biggest reasons for delving deep underground
to begin with. However, as we learn more geology and get
better at modeling complex systems like weather and tectonics, we might be able to learn to
predict earthquakes and volcanoes and might be able to stop them too. It’s all about pressure and releasing it
after all, and the cool thing about some vent you’ve bored into the ground to release
pressure is that it also makes for one heck of an awesome place to put a turbine to generate
power. Heat free power too, the best kind for mega-civilizations,
because it’s already heat that exists and you’d have to get rid of it anyway. So we can definitely see a lot of reasons
why we’d build many things underground, and that would result in a fair number of
people living there even if it was mostly automated, but one last pathway of note is
the iceberg style of housing. In a mostly automated and power-rich culture,
it’s very easy to imagine that vast underground regions wouldn’t be limited to megacities
and giant skyscraper arcologies, using the underground for storage, manufacturing, and
agriculture. You could also have a lot of relatively small
individual houses, on the surface, for a single person or family, that had huge underground
sections they used for less day to day living. Attics and basement get full of knick knacks
as one ages and in a high-tech civilization, a person might live many centuries and accumulate
a lot of stuff. Such places might be where you kept that,
where you kept the extra guest rooms, where you kept the hydroponics or lesser manufacturing
your own home did rather than getting it delivered, like a food printer, or your own power plant
or water treatment facilities and their storage tanks and cisterns. Essentially an iceberg house, where just a
little bit peaks up with far more below, if tunnel boring got cheap enough, you might
even have a tunnel to your house rather than a roadway, power and telephone lines, etc. So we see there are a lot of advantages to
building down, a lot of things that could be done there to save space on the ground
for other things, and many ways to make the Earth’s depths a lot more earth-like or
interesting in their own right, like mushrooms forests or twilight seas beneath a stalactite
sky. All of that said, while I can see us building
underground a lot in the future, especially on other worlds, I still don’t see most
folks living underground. Though I do think many would and certainly
many businesses or tourists spots might opt for the depths. An exception to that might be if we go the
extreme route and star building whole new layers of planet that are fully lit, which
we’ll get to later in the series. First though we’ll look at an option with
a better view, the kind you could only get from a mountain or a plane, in Cloud Cities,
next time in the Earth 2.0 series. If you’re building tall buildings or excavating
deep ones, it’s important to have a strong foundation. The same is true of knowledge and to really
grasp a lot of the concepts we discuss here and learn more on your own, you need to make
sure your knowledge is resting on a strong foundation too. At Brilliant, you can improve or refresh that
knowledge with many well-designed and easy to use courses and quizzes that let you start
where you should and proceed at your own pace, from the basics of math and science all the
way through far more advanced levels. If you’d like to strengthen your knowledge
and build on that foundation, go to brilliant.org/IsaacArthur and sign up for free. And also, the first 200 people that go to
that link will get 20% off the annual Premium subscription. Next week we’ll be exploring a popular concept
in science-fiction, the notion of planets given over to prison colonies, and we’ll
look at how realistic that is. The week after that, we’ll look into how
life might have originated on Earth, or rather the possibility that it might not have originated
here but out in space, in the Fermi Paradox & Panspermia. For alerts when those and other episodes come
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have a Great Week!
Antarctica would be a good place for underground or under ice settlements. One particular place would be the transact arctic mountain range. Lots of minerals to exploit, lots of cold for heat exchange. https://www.en.m.wikipedia.org/wiki/Transantarctic_Mountains
Was that a Starship/Heavy in the shot when Isaac was talking about the cloud city?
Lovin the vids. Please do a video examining what changes we need to make to academia to realize these futures.
What is his accent?
Only the first few kilometers of the crust are going to be accessible with any kind of conventional technology. What if you want to deeper though, into the mantle? I've seen science fiction stories which do that, but that would probably require supertechnology, force fields or something.
I loved the joke in the episode!
I've been collecting stories of undergrouns cities, etc., for nearly 40 years (since the age of 12). Here's a link to a database containing many of them that I have put on the web, but there are still MANY that I haven't even had time to put in this database yet, because of time restraints: http://www.angelfire.com/ut/branton/redbook1.html