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In the future, a high-tech human civilization has colonised a distant planet. The planet is basically Earthlike in gravity, climate, terrain and vegetation. They want to build Forevertown: A city with low-tech construction which is built to last.

The projected lifespan of Forevertown is measured in millions of years. To start off with, the inhabitants will have high technology; but the city should remain habitable if they regress to Iron Age or (possibly) Stone Age levels. For the sake of argument, assume the city will not be deliberately destroyed by human action.

The technology of the city builders is highly advanced but not "magical" by our standards. They can't violate the laws of physics as we currently understand them; in particular, magic self-repairing materials are not an option over this timescale.

What is the best way to build Forevertown? More specifically:

What form of construction would be most durable, with least maintenance? Mortarless stone buildings as seen in Machu Picchu are an obvious candidate, but I am open to other suggestions.

Related questions:

What is the best location for Forevertown?

What are key maintenance issues for Forevertown?

Edit: Mind-boggling though these timescales may be, the Earth is projected to remain habitable for about 1 billion years. If there are still humans then, and they haven't fallen victim to the Singularity / zombie apocalypse / demolition of Earth for hyperspace bypass, they will have to confront similar issues over the very long term.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monica Cellio
    Mar 22, 2015 at 19:19
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    $\begingroup$ This might be of relevance to parts of all your related questions: Go read Deep Time by Gregory Benford, or watch Into Eternity, about how to store nuclear waste for a mind-bogglingly long time. Granted, your scenario has people living in the area for all that time, as opposed to these. $\endgroup$
    – KlaymenDK
    Mar 23, 2015 at 14:33
  • $\begingroup$ Accepting Serban Tanasa's answer as I think it's the best overall, but thanks for other answers, there are some very good ideas in there! $\endgroup$
    – Royal Canadian Bandit
    Mar 26, 2015 at 10:06

18 Answers 18

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Granite is one of the most durable types of rock. As you walk across a granite staircase, the electromagnetic fields in your shoes are repelled by the electrons in the outer layers of the outermost atoms in the granite slab. Occasionally, a few granite atoms are kicked loose in the interactions. Over the centuries, this builds the grooves you see in old palace staircases. Same thing happens with every drop of rain that lands on the Machu Picchu, every gust of wind that brushes against the Pyramids of Giza.

The point is, 10 million years is a very, very long time. It is about 2000 times longer than the Pyramids of Giza have been in existence, and about 100 times longer than biologically modern humans have lived on this planet.

Furthermore, the pyramids were mostly a solid set of blocks with little inner structure, definitely not meant to be lived in, and located in a dry inhospitable area. They have not aged gracefully:

Giza "Look on my works, ye Mighty, and despair!"
enter image description here
"Nothing beside remains. Round the decay
Of that colossal wreck, boundless and bare
The lone and level sands stretch far away."

Any functional city has many moving parts, and moving parts wear out. Magnitude 9+ earthquakes can crack and twist the landscape itself, divert rivers and crush stone. Rivers can carve stone over the aeons. Temperature variations can microfracture stones. Moss and fungi can grow on the smallest crack in the hardest stone. Dust particles deposit and build themselves to tens of meters. Lands can be submerged, or sunk beneath the sands. Ice-ages can layer 3 kilometers of rock-hard ice on top of your structures and grind them into dust.

So, what do all these reasons it can't be done that tell us about how to do it?

  • Build in the most geologically stable area you can find.
  • Embed directly into the bedrock, preferably by creating a 20-km wide artificial bedrock (or even better, an artificial composite material with the hardness of diamond and the strength of aramid fibers)
  • Build in a dryish climate, far away from easily flooded areas in case of sea level change and out of reach of glaciations. Have your nearby sources of water follow a prebuilt (high-strength) canal, with 1000-fold spare capacity to prevent it from ever flooding.
  • For your buildings use a nonreactive nanocomposite core (similar to the kind we use in tanks and aircraft) to be covered in low tech granite slabs that can be replaced.
  • Use a massive, heavy architectural style. No fragile high-flying arches for your city.
  • Have both the city as a whole and individual buildings be highly stabilized against earthquakes by placing them on a durable version of bearings.
  • Create a spaceport, where a von Neumann self-repairing machine that spends virtually all its time in space gathering energy and raw materials can return every 1000 years to release bots that will conduct maintenance.

Still, your biggest threat will not be the elements but humans. The great Pyramid of Giza once had a shining facade of white stone and a gold pyramidion, that made it a splendid sight from tens of miles away. All of this was looted, and all we can see today is the inner structure. Looters will be by far your biggest concern, and unless you want to keep gamma-ray robots in orbit to blast away the blasphemers, there's little you can do about that...

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    $\begingroup$ And what's wrong with orbiting anti-blasphemer death rays, I ask you? ;-) $\endgroup$
    – Royal Canadian Bandit
    Mar 19, 2015 at 14:59
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    $\begingroup$ Nothing, of course (looks up, with worry). Praise be unto the most holy flying spaghetti monster! $\endgroup$
    – Serban Tanasa
    Mar 19, 2015 at 15:03
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    $\begingroup$ Pick a geologically dead planet (no molten core) -- no earthquakes, no continental drift. $\endgroup$
    – Dan Pichelman
    Mar 19, 2015 at 20:09
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    $\begingroup$ @DanPichelman, no Carbon or Nitrogen cycles either. Dead planet indeed. $\endgroup$
    – Serban Tanasa
    Mar 19, 2015 at 20:25
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    $\begingroup$ Good to see you covered the looting of the limestone casing of the pyramids. In fact, the lower courses were granite covered; this was left in place. (Does not come out well in your images). The lesson is: use materials that aren't useful to looters - undeniably a challenge given the valuable durability characteristics. $\endgroup$
    – Keith
    Mar 20, 2015 at 2:19
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magic self-repairing materials are not an option over this timescale.

I know you said this, but I'm assuming you're talking about stuff like self-repairing nanomachines. But there's an alternative, very non-magical answer to this question that's also self-repairing: Plants.

It would require genetic engineering, but is still something I think would be reasonable. You'd modify trees so they created human-friendly structures - plantable walls, enclosed ceilings, etc. Then you can have a living city, with an indefinite lifespan. The technology is all in the creation of the plant, so even if your people regress to the stone age they can still collect seeds and re-plant buildings. Depending on how crazy you want to get, you could even create the plant so it turns some of the sunlight it collects into electricity and provides it from sockets that grow into the walls (or that can be accessed with mechanical sockets).

There are two potential major downsides to this, but I don't think either is insurmountable:

  1. Disease. Since your plants are alive, they can presumably, theoretically be attacks by things that attack other plants. An engineered plant could be extremely resistant, but your inhabitants might need to keep some basic knowledge of "burn diseased buildings and replace them with new, uncorrupted seeds".
  2. Mutation. You're asking for a literally evolutionary timescale, and it's possible the plants will slowly adapt. However, I think human activity will, for the most part, guide this evolution in useful ways. Plants that grow out of control will end up being pruned or burned.
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    $\begingroup$ I had some thoughts along these lines, but they had developed enough to be cohesive. This sounds plausible. Note that nutrition for the plant over climate changes becomes an issue however. $\endgroup$
    – Rozwel
    Mar 19, 2015 at 20:12
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    $\begingroup$ Interesting point, and yes I was thinking of nanomachines rather than living organisms -- although a sufficiently advanced technology would not necessarily recognise the distinction. ;-) $\endgroup$
    – Royal Canadian Bandit
    Mar 20, 2015 at 10:05
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    $\begingroup$ This is nice, and we actually have non-genetic engineered versions of this! Checkout "living bridges": en.wikipedia.org/wiki/Simple_suspension_bridge#Living_bridges Some are really amazing - it's just that they have been continually adjusted by caretakers, in a tradition passed down for hundreds of years so far (and no one destroyed the things intentionally, of course). And we do have 80,000 year old Aspens, so it's a start! smithsonianmag.com/science-nature/… $\endgroup$
    – BrianH
    Mar 20, 2015 at 15:28
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    $\begingroup$ If you're genetically engineering something that complicated, there's a good chance you could "turn off" mutations, by including some sort of genetic checksum and having cells die if the checksum is wrong. $\endgroup$
    – Brendan Long
    Mar 20, 2015 at 20:05
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    $\begingroup$ @mafu But evolution works because tiny mutations add up over time, not because a bunch of mutations happen at once. If you could prevent any small mutations from happening, then you'd essentially end evolution for that species permanently. For a trivial case, example, you could duplicate the entire genome and kill the entire cell if they don't match. Since the genome of something like a human is several billion base pairs long, the chances of two genomes being the same after a random mutations is around 1 in several billion. Duplicate it twice and the odds go to 1 in several quintillion. $\endgroup$
    – Brendan Long
    Mar 21, 2015 at 16:16
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10 million years is a geologic-level time scale - so if you truly want a structure to last the ages, then we're going to need some help...specifically, from geology itself.

First, let's consider just how long 10 Ma really is:

Fun geologic timeline

So we need something that will last approximately three Epochs...wow, this is going to be epic!

As a matter of fact, this is beyond human time:

Human ancestors timeline

That's right, this is about 5 times longer than our closest biological ancestors were even alive...no homo. (rim shot, please, Paul)

But we're in luck because there indeed structures that are approximately this old, or even older - caverns and cave systems!

The Jenolan Caves are considered to be hundreds of millions of years old, but they are also very wet, which will tend to make keeping things intact likely more difficult than in drier conditions. The Grand Canyon Caverns claims to be much younger, perhaps only around a few million years old in its present form, but it's also dry! Water is so good at eroding things, that I'd likely want to pick such a place to build - you can always drill to get access to the water table if you want, and it's easier to control than having it leak in all over the place.

The beauty of caves is that they provide a world that seems relatively unchanging compared to the surface - as long as it doesn't get buried beneath mile-thick ice sheets during an ice age, or have the tectonic plates shift so much that it screws the place up by flooding it or filling it with toxic/explosive gases, you'll be fine!

The natural environment of caves is so good at preserving things that things that died tens of thousands of years ago leave perfectly nice collections of bones. In some cases, the preservation is so good that scientists have managed to extract significant amounts of DNA from animals that have been dead/extinct for many thousands of years - and no one was even trying to preserve these things! The work of the pyramids seems so small compared to what our natural environment can do all by itself sometimes, doesn't it?

This doesn't necessarily have to be a city for mole-people, though. There's no reason such a system couldn't be worked to have a number of "natural entrances", and people could conduct all sorts of outdoors activities. But if such a place existed there would be a strong incentive to develop underground living to a fine art and science, though.

What caves buy you is something you need - protection from the elements. Otherwise, we don't really have any non-exotic substance that can last so long against the weather alone, and if we did it would need regular refurbishing/maintenance that is going to require high technology.

What I like about caves is that progressive levels of technology could be needed to work/live/survive deeper and deeper recesses of the caves. If your people 'forget' how to make the mixture that goes into the UV light fixtures, or maintain the clean air systems, or operate the airlock/airway systems that allow access to the mines or geothermal vent forges, or stop maintaining the water-control systems so they flood lower levels...then the city doesn't get wiped out! They just begin to lose access to the higher-technology awesome stuff that is only possible to enjoy with higher technology.

In theory, even an ancient people with little or no technological understanding could live in the cave systems, even if they have become unable to do the things necessary to navigate, survive, and fully enjoy subterranean living.

With some maintenance, only your imagination is the limit - there are no unsolvable problems deep down. You can grow things, mine things, gather raw materials...the technology level used would be up to your civilization to determine. Want robotic-level automated facilities? Have them conduct their own maintenance, and perhaps only leave material gathering to humans if you don't want robots running around topside. Have big, gorgeous diamond/sapphire-coated representations of the raw materials needed by the deeper inner-workings to label the "inboxes". The humans just put the iron ore in the bin, wood in another bin, bones in another bin, etc, and the system handles the rest.

How you want your story to go would, of course, depend on how the system was crafted and what the exposed/moving parts would be. A semi-autonomous automated system isn't completely out of our current technological abilities, so if this was built by a more advanced race they'd certainly be able to figure out auto-factories that can keep themselves going so long as they are provided with acceptable parameters. Make these inputs or environmental parameters the rules of the city, and so long as the inhabitants keep up their end of the bargain the system works. If they stop the system can have cascading levels of shutdown, ultimately having a core that survives on only the smallest amount of inputs, and designed to safely put itself in permanent hibernation if the final ingredient is no longer provided (your pick on what that might be - nutrient solution, sugar, blood, electricity, light, radioactive materials, whatever).

The biggest danger will be dealing with people, then, screwing with things they shouldn't or not following the rules. Geological changes will cause issues, but an advanced race should be able to understand/predict with some reasonable degree of accuracy what might happen and plan a location or operating parameters that would deal with such a problem - they'll avoid systems that can be flooding with a shifting water table or areas likely to be buried under ice sheets unless the system can support people with no access to the outside, etc. Could be like a big bomb-shelter, or they could just ensure the build was in a place that they felt wouldn't face this problem.

One note, though - over this period of time the people who first lived in the city or designed it will be different biologically over this time period. If people live inside most of the time or even all of it, there will surely be significant biological and psychological/culture changes over such a stretch of time, and so the people might be as different from their origins as we are from our non-bipedal ancestors (walking on two legs is believed to have started around 6 million years ago in human ancestral lines).

That said, there's a crazy amount of possibilities there, and none of it requires magic! How you'll keep people from blowing it up, drilling into the wrong parts, etc, I don't know - but that's not the question. So enjoy!

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  • $\begingroup$ +1 Can't help but imagine being trapped under the earth for 10 million years after an earthquake. Shudder. $\endgroup$
    – Serban Tanasa
    Mar 19, 2015 at 23:16
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    $\begingroup$ There is some hindsight bias here - Looking back, we would find really old caves that preserved well - but could we do the same looking forward? Which of the caves on earth today would survive the next 10 million years? $\endgroup$
    – mrwaim
    Mar 20, 2015 at 9:30
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    $\begingroup$ @mrwaim That's a very good point! To be able to pull this off successfully you'd need at least a very good understanding of all the physics and geology involved, and it's very unclear as to whether or not these are "chaotic" processes that can't be predicted. In such a case either it's a crap shoot, you'd have to plan for things that could go wrong (like flooding), or you need to build a number of cities the same way around the world with the expectation that they won't all necessarily survive. $\endgroup$
    – BrianH
    Mar 20, 2015 at 15:24
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    $\begingroup$ Those caves would not have lasted nearly that long if there were people shuffling through them every day. $\endgroup$
    – BlueRaja - Danny Pflughoeft
    Mar 20, 2015 at 19:48
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A million years is a VERY long time. Mountains wear down over millions of years.

The oldest structures we made is in the 5-8000 year old that is still standing. Stonehenge is about 5000 years old. The pyramids start about 4500 years ago. Here is a list of oldest buildings as we know them. The oldest currently is less than 7000 years old.

Stone is the most resistant but a million years is plenty of time for wind and water to cause it to wear away completely. Stone steps that are only a 100 years old have worn groves from feet. A million years of rain and wind not to mention ice will leave nothing standing. This all doesn't take into account any plate tectonics. Earthquakes will speed it up, and don't forget the changes in climate. Ice ages come and go, jungles migrate around the globe, volcanoes bury things under ash, winds move sand and dirt.

For it to exist for a million years something would need 'constant' maintenance if even once a century. At that point anything could last.

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    $\begingroup$ I think even only erosion by humans is going to be an issue. Just look at stone stairs or railings in monuments merely a couple centuries old (churches and the like), they tend to be rounded by people stepping on them. Now imagine a factor 10'000 of that and I wonder how much of the stone will be left. $\endgroup$
    – drat
    Mar 20, 2015 at 1:25
  • $\begingroup$ Stonehenge wasn't still standing, actually the the tops fall off and people put them back on. Also they gave some of them concrete bases to stop them falling over. $\endgroup$
    – RedSonja
    Mar 23, 2015 at 11:42
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An alternate approach, which seams to run counter to the scenario in the question, is that they don't plan for the individual structures to last forever.

Instead we design a city with structures that are reasonably durable and can realistically be expected to last a generation or two. The materials, or at least surface layers, are low tech, and easy to produce/recycle within the local environment. Under this scenario the inhabitants are constantly rebuilding or refinishing structures to keep them habitable. No one building lasts forever, but the settlement lasts as long as people are willing to stay there and take care of it.

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There is something in this question that reminds me of "how do you turn base metals into gold?" in that it presupposes there is an answer, and also that the answer is technological.

The key thing about cities is that they are places for humans, and humans have needs beyond mere shelter. How are you going to satisfy those needs - for food, water, warmth? I guess there are technological solutions to those, but at that level you are only surviving, not thriving. If that is all you cater for, it is not safe to assume that some of those humans will not set out to destroy the city.

What makes a city thrive is the dynamics of the social interaction, and somehow addressing the various social problems that arise. Social problems need social solutions, or at least socially-minded solutions, not purely technological ones. Technology applied to social problems without understanding the deeper social issues can be like calming the frog in the pot, but not minding that the heat is steadily raised towards boiling point. There is a problem in thinking the frog belongs in the pot at all, and likewise in assuming you can possibly solve social problems with technological advances.

Now if you want to talk about Forevertown being a village rather than a city, such that the scale of the social issues and interactions is much more manageable in line with the principles of Dunbar's Number, then maybe you have a chance. But if you're prepared to consider that, then I'd encourage you to stop discussing imaginary solutions, and get out there in the real world where long-term peaceful sustainable living is a real and pressing challenge.

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    $\begingroup$ Telling SE participants to "get out there in the real world" is not relevant to the question, and to put it mildly is likely to be unproductive. I suggest you remove it from your answer. $\endgroup$
    – Royal Canadian Bandit
    Mar 20, 2015 at 9:25
  • $\begingroup$ Please re-read - you'll find that I didn't "tell", I encouraged, and did it in context - a context which I believed would end up providing more grounded and realistic answers because it stood to shine considerably more light on the true nature of the question. $\endgroup$
    – omatai
    Mar 20, 2015 at 9:43
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    $\begingroup$ "Tell", "encourage", call it whatever you want. It's still not relevant, and quite frankly it's very presumptuous since you have no idea what real-world experience the other participants on this site may have. You can discuss relevant aspects of smaller communities perfectly well without "encouraging" anyone to do anything In Real Life. $\endgroup$
    – Royal Canadian Bandit
    Mar 20, 2015 at 9:46
  • $\begingroup$ I can't stop you interpreting my answer in ways I didn't intend any more than you can stop me interpreting your question in ways you didn't intend. It happens. Ironically it underscores the relevance of social factors in human society that certainly have not been overcome by the technological "solution" we are using!!! $\endgroup$
    – omatai
    Mar 20, 2015 at 10:30
  • $\begingroup$ "how do you turn base metals into gold? Done. We didn't repeat it as there was no why besides proving we could. $\endgroup$
    – Joshua
    Mar 20, 2015 at 17:16
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What can easily survive 10 million years? - a mountain of course. So build one and make it habitable.

Pick center of biggest tectonic plate - it will last longer than 10 millions and not too many earthquakes. This will ensure that granite won't crack.

Build a huge granite circle: with radius of 10 km and 2 km thick.

In the middle build a crater 0.5 km deep - this is where people are supposed to build houses.

Crater can have some nice terrain inside as well. Crater will protect people from winds, and has lake inside - to be source of water, rainfall goes to lake, oveflow leaves the city.

Make sure to build some "rivers" to remove water and the yucky stuff from crater.

Also city terrain should not have any pits, so all the water and wreckage can leave the city naturally. Lake is very flat by default and is made so that it can be easily raised in level by mini-dam.

Water and cleaning: city is it's own source of water so they just poop into one of the "rivers".

Since people are not supposed to destroy the city - we do need to worry that they will end up mining it. And even if they do, 50+ cubic kilometers of stone is quite enough.

And population limit could be ~100k.

If ice age comes city will be chilling without people until ice age ends.

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  • $\begingroup$ You almost make living in a crater sound fun! Until the part about the bathroom situation $\endgroup$
    – Radvylf Programs
    May 27, 2018 at 3:18
  • $\begingroup$ bathroom stuff with trickle down out of the crater $\endgroup$
    – Bohdan
    May 29, 2018 at 22:58
  • $\begingroup$ True. I guess you're right $\endgroup$
    – Radvylf Programs
    May 30, 2018 at 2:09
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I believe the only type of artificial habitat that can remain for a million years would be to dig tunnels and chambers into solid rock. If they are located far away from tectonic plate edges in regions with very low tectonic activity, there is very little that could cause them to collapse.

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I don't think this is possible out of current knowledge and theory. At least not if there is significant technological regression.

To last "forever" a structure has to be able to resist wind, rain, fire, flood, earthquakes, freeze/thaw cycles, mold/fungus/plants, animals/insects, along with wear and tear from its occupants. On the time scale you are talking about you also have to be able to handle multiple climate and environmental changes which makes the problem even more difficult. Almost always, materials that stand up well to one or more of these are quite vulnerable to others.

In theory, if you could come up with a material that was completely non organic, resistant to impact and abrasion, resistant to oxidation and other corrosive processes, non porous, and strong enough to hold up its own weight and that of its occupants, you might have a chance. Everything I know of today relies on combinations of materials and regular maintenance to resists the various hazards. Most such maintenance requires high tech at some level of preparation or application.

I think your best bet lies in the realm of a formed rock. Essentially casting/fusing the stone so that the structure is one solid piece. The surfaces have to be perfectly sealed, with no cracks or pores down to the microscopic level. You can not have any hard angles, things have to be shaped such that vibrations and applied forces flow without having places to build up/reinforce and cause fractures or fatigue. Moving parts are also most likely out, so doors and windows may just be, or end up as, open holes that the occupants fill with whatever materials they have the technology available to work with. Even so I think your useable life is probably measured on the scale of thousands of years rather than millions.

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    $\begingroup$ I said "least maintenance" in the OP, not "no maintenance". It's fine if the inhabitants have to replace parts of the structures from time to time, it just has to be feasible with low technology. $\endgroup$
    – Royal Canadian Bandit
    Mar 19, 2015 at 14:04
  • $\begingroup$ My main concern is technological regression. I don't think it is feasible for a stone or early iron age culture to be able to perform the necessary maintenance. $\endgroup$
    – Rozwel
    Mar 19, 2015 at 15:57
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Diamond/ carbon nanotubes have the property of being combustible, so perhaps they are not a good choices. I think quartz crystal might be more resistant to erosion than granite and supposedly does not have a big issue with fragility due to cleavage planes.

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  • $\begingroup$ Indeed, while advertisements claim that diamonds are forever, actually they are only metastable under normal conditions. However while the information that it is metastable is all over the net, I've not been able to find any information about the lifetime, so I don't know if this would be an issue in a time frame of ten million years. $\endgroup$
    – celtschk
    Mar 21, 2015 at 11:27
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    $\begingroup$ Diamonds are thermodynamically unstable and will sublimate (for lack of a better term) over time. Oxygen attacks nanotubes pretty effectively unless they use something like Florine as the outer atom. $\endgroup$
    – Jim2B
    Mar 23, 2015 at 3:25
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I don't think that it is fundamentally possible because any habitable planet will have geologic activity as well as climate actions.

Earth as a planet will remain habitable for a very long time, but no individual spot can be guaranteed to remain habitable.

Over 10 million years, you have to look at such things as an ice age steamrolling over your city regardless of how it is built. You have to look at continental plates crushing and in the process changing the shape of a formerly flat area into vertical mountain cliffs or even turning your city upside down. You have to look at the crust of the earth being stretched and possibly a new ocean forming where you built your "forever" city.

Of course you can TRY to find the most geologically stable spot, but pretty much no matter where you go, over 10 million years, your town will suffer catastrophic events that even the rock itself can't withstand.

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I'm not certain why "regression" is even something worth worrying about. A culture is going to build a structure for the purposes of its own culture (or at least one reasonably similar to it) so I don't at all see the point of making something able to survive with minimal tech/knowledge. Maybe if the question was more how a benevolent Demi-God-like race would build an everlasting protective structure for a less developed race.

Besides, it seems to me that the whole civilization dying out is much more likely than the people regressing that far as a whole. Just because that has been a popular event in literature doesn't mean that such an event is at all likely. Especially in a civilization in which, presumably, their tech and understanding is much greater than our own. I see the chances of some disease wiping out the higher functioning power of an entire species (yet not killing them) unlikely given the tech, knowledge, and all the pressupositions that go into the ability to even create Forevertown. So I would say you remove any precautionary planning of species regression.

Even more likely than extinction (or regression), would be the civilization simply leaving to anothor habitable space, abandoning Forevertown. Forevertown 2.0 is a MUCH better iteration of its first attempt, especially if it was built by Apple ;). If Forevertown's culture is able to create such an amazing thing as Forevertown, give them a few more thousand years (or half a million) and planet building will be their next venture. Ring World anyone (Larry Niven's books)? In the end, I just can't imagine a scenary in which such an everlasting immense structure would be useful (return on investment, economics, conservation of resources...) compared to a civilizations more natural way of doing things. Constructing and re-constructing seems to work better. Or better yet, building and abandoning. I think maybe a giant monument to a civilization's prowess, or a timecapsule of sorts would be a more useful idea.

I realize this isn't exactly answering the question, but I think it is fair to at least say this probably wouldn't be a useful enterprise for any advanced civilization.

My actual answer to this question is simply that it would be a bad idea. Any other answer I could think of would be incredibly limiting to the inhabitants. Even if there were magical materials that would never wear or change even if passed through the sun, such a substance would be pretty hard (or impossibly) to modify for the simplist of changes. I would rather not force a whole population to live in an unalterable city. From small towns to massive towns in the U.S. there is immense change over 100 years. Often times it is indistinguishable. The needs of a city and culture change so much it is impossible to build a permanent structure of any size that won't inhibit improvements.

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Your best option for building material would have to be diamond but we're talking shells of structures here - habitable, yes but after millions of years everything else inside the building will have decomposed to uselessness. No lights, electrical systems, lifts, hinges, etc. gone.

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    $\begingroup$ I have to disagree here. Diamonds are resistant to scratching and heat, but they are a crystal and shatter fairly easily with an impact. One storm with heavy hail or blown debris could conceivably leave your diamond shells a pile of rubble. $\endgroup$
    – Rozwel
    Mar 19, 2015 at 20:28
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Very strong or durable materials... nothing. But knowledge may be durable. Then, they will constructed of very simple materials (like sand or vegetates), but with the acquired knowledge to replicate forever. We can analyze the oldest buildings, like the pyramids of Egypt or Stonehenge. But if we have lost the knowledge of were it was be made, we can only see them (and perhaps to guess its technology). Another point of view is analyze the most older cultures remaining in the world. Check a pre-Incan people who live on forty-two self-fashioned floating islands in Lake Titicaca Puno and Lake Uru Uru. The Uros use bundles of dried totora reeds to make reed boats (balsas mats), and to make the islands themselves. The larger islands house about ten families, while smaller ones, only about thirty meters wide, house only two or three. The islets are made of totora reeds, which grow in the lake. The dense roots that the plants develop and interweave form a natural layer called Khili (about one to two meters thick) that support the islands. They are anchored with ropes attached to sticks driven into the bottom of the lake. The reeds at the bottoms of the islands rot away fairly quickly, so new reeds are added to the top constantly, about every three months; this is what makes it exciting for tourists when walking on the island. This is especially important in the rainy season when the reeds rot much faster. The islands last about thirty years.

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You want to build your city in space. Find a sufficiently large asteroid. Hollow it out, make it rotate, live on the inner surface. Plenty of science fiction authors have given rich descriptions of what such habitats can look like. If designed properly, there really isn't anything that stops it from lasting ten milllion years.

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    $\begingroup$ Except that living in an asteroid probably requires all sorts of advanced technology (solar power collectors, nuclear power, CO2 scrubbers, etc.) that likely will not survive the kind of technological regression that the OP presumes. $\endgroup$
    – jschultz410
    Mar 22, 2015 at 18:42
  • $\begingroup$ I agree, a planet's weather systems would preclude any structure surviving millions of years. The only place we know structures survive is on the surface of airless & dead planets/planetoids (e.g. the Moon). Although I agree that with our current state of knowledge we could not build a survivable space habitat for this period of time, imagine a giant space terrarium - filled up with a biosphere, set in motion, and sealed up from the outside. No active maintenance required. This is possible but there would still be problems. How do you keep the human population in check? $\endgroup$
    – Jim2B
    Mar 23, 2015 at 3:29
  • $\begingroup$ @Jim2B The last problem already exists right now. $\endgroup$
    – gerrit
    Mar 23, 2015 at 15:38
  • $\begingroup$ True, but as in several SF stories already (ala Logan's Run), it could add interesting dynamics to your space terrarium. Also it need not be in a planet, moon, or planetoid. $\endgroup$
    – Jim2B
    Mar 23, 2015 at 18:32
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OP mentioned that occasional repairs are acceptable, as long as they can be made by using simple technologies after technological regression.

For such requirements, mud bricks are perfect fit. Build it in dry climate, and maintain it yearly: Timbuktu and Djenné. If climate changes, city can be rebuild few miles upriver every generation or so.

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Even today we can create some pretty amazing materials. And a space-faring civilization would probably consider our crude "high-tech" materials as we consider clay of stone age people.

So, extrapolate from modern high-tech materials. Simply make the town of materials, which will not be eroded or corroded away in 10 million years. Have nanotech construction technology to place desired alloys and other materials in a composite matrix. If cost is an issue, the material should probably be based mostly on carbon, silicon, iron, and nickel, but a space-faring civilization would presumably have an abundant supply of all exotic elements, by asteroid mining.

An important consideration is architecture. You don't want water to accumulate anywhere, even if something clogs drainage systems. You want weaker spots (windows, probably) protected from the weather. You want to make people walk on areas which are built thicker to take the increased wear. And if course you want everything thick enough so that it can take 10 million years of wear with enough thickness left (50%?), and most wear concentrated on surfaces where it doesn't affect functionality.

If you want to get technical, compare the material to modern materials, either in actual text or in an Appendix depending on style. Then let the reader extrapolate in their own imagination. Current advanced materials are mostly made for extreme environments, such as turbine blades of jet engines, disk brakes of sports cars, pump parts for manufacturing highly corrosive chemicals, and so on. On the other hand, advanced construction materials of today are sensitive to cost, which limits them. A material designed to last "forever" in "normal" conditions with little worry about cost would probably be optimized a bit differently, using rare elements freely, yet not for extreme conditions. For example, resistive more against wear by raindrops than wear by molten lava. So you don't have to and you shouldn't pick an existing alloy/composite, just use them for ideas.

Some links to get an idea

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  • $\begingroup$ "Make it out of unobtanium" isn't a very good answer. Ten million years is enough time to carve the Grand Canyon or raise the Cascade volcanoes. We don't even have hints of what a material that could last ten million years without being eroded or corroded would be like. $\endgroup$
    – Mark
    Mar 22, 2015 at 5:54
  • $\begingroup$ @Mark This is "material like we do today, except implemented with use future technological improvement". That isn't unobtainium, at worst it is handwavium. And of course we have "hints" of what could last 10 million years, as we need to know how often the parts made from current materials needs to be replaced before chemicals start to spill from factories and planes start to drop from the sky (more often than these things happen today). Scaling down the wear to "normal weather conditions" and larger material thicknesses shouldn't be too hard. $\endgroup$
    – hyde
    Mar 22, 2015 at 6:06
  • $\begingroup$ @Mark I am no material engineer, so I can't give exact figures, but since this angle, extrapolate from current materials, wasn't covered by other answers, I felt it was valuable answer. But perhaps I delete it then. $\endgroup$
    – hyde
    Mar 22, 2015 at 6:07
  • $\begingroup$ I've got a materials-testing background. Over a timescale of millions of years, you're going to encounter effects that are generally ignored as being smaller than the rounding errors in current test procedures. I'd expect effects like metal fatigue from day-night thermal cycling or migration of individual atoms within an alloy to become significant on those scales. $\endgroup$
    – Mark
    Mar 22, 2015 at 6:23
  • $\begingroup$ A cylinder of ordinary car can easily go through, roughly (500 Mm / 50 km/h * 60 minutes * 2000 rpm) 1,2e12 thermal cycles without needing to be retooled (feel free to re-check my math, I did it only once). 10 million years is just about 3e9 day-night thermal cycles. Migration of atoms is certainly a possible issue, but not something that couldn't be accounted for, and actually taken advantage of (for example to help maintain perfect composition of the eroding oxidation layer at the surface over time). $\endgroup$
    – hyde
    Mar 22, 2015 at 6:51
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What has lasted billions of years? Oceans. It is fairly certain that oceans aren't going anywhere any time soon. Imagine a huge boat, made of aluminum. Immediately, you can find a problem with this: storms. A storm could quickly destroy or flood a boat. Solution? Root it into the ground, like an oil rig. However, tsunami waves could destroy this easily. But if it were halfway underwater and ha domed roof, it could withstand tsunamis. To allow people to walk out and see the sky, replace the dome. Instead, you could have a set of aluminum panels that are set on the outside of the ceiling of the huge city, and use replaceable bolts to lock it in place. As long as the people are careful not to scratch the panels, no water will get in. As a bonus, soil could be placed on top of the city, allowing for a "park", which would be regrown after huge storms. This may not last 10 Ma, but it will last a while.

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  • $\begingroup$ Without active maintenance, this city will be hard-pressed to last ten years, much less ten million. The ocean is an extremely hostile environment; even if your floating city doesn't get destroyed by storms, it will get encrusted with marine life and corroded by saltwater. Over the course of ten million years, even the ordinary action of waves lapping at the sides will be sufficient to reduce it to a pile of powdered corundum. $\endgroup$
    – Mark
    Jul 18, 2019 at 2:05

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