What kinds of remnants might be evidence of a previous civilisation?

Question

The trope of an ancient advanced civilisation that disappeared is as old as the hills, but it does raise an interesting question: what kinds of evidence would suggest that somebody else was here first? After a few hundred years, most buildings, bridges, and other structures would have collapsed, and after a few thousand there are likely to be only fragments. After a million years or more, is anything likely to remain? Humanity has made such a huge impact on the Earth that even a million years after we're gone, surely something will remain; I can't think of a way that buried reinforced concrete "rocks" could've formed naturally, for example.

I'm toying with the idea that a prior, sufficiently advanced, civilisation disappeared in some kind of cataclysm at least a hundred thousand years ago, preferably more. I don't want there to be any ruined buildings or monuments, but I'm ok with the extremely rare magical artifact. I don't know exactly what kinds of technologies and abilities these peoples had, other than that they were probably equivalent to a Kardashev Type I or II. I do like the idea that they were capable of performing geological scale engineering or terraforming because that's a nice little deus ex machina to conceal my map-drawing abilities—the in-universe hint would be that the dates from radiocarbon dating and supposed geological timescales required for mountain building just don't add up.

On the surface, it feels like lazy worldbuilding, but I want to turn that trope on its head somewhat. To prevent this question from being too broad, I want to limit the evidence to be geological in origin (so if there's a partially-complete Dyson sphere around the sun, it hasn't been discovered yet).

Answer 1

There are some long lasting footprints a civilization can leave behind:

• Buried glass manufacts: unless in contact with fluoridric acid, glass is practically untouched by other reactant. This, together with the peculiar shapes we give to most glass objects (flat sheets for windows, hollow bottles or glasses, just to say some) is a clear sign of non-natural process behind the formation of those artifacts
• Large engineering jobs: think of nuclear plants, or large dams (a K-I society should have them). What they leave in the place they are built is significant and very likely to be noticeable even after very long time. Not only the concrete foundation, but also, for the nuclear plant, residual background radioactivity or, for the dam, different erosion/sediment layers.

Just as an example, we have been able to find a natural fission reactor 1.7 billion year after it was active.

Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging probably less than 100 kW of thermal power during that time.

Answer 2

Radioactive isotopes.

If there was any nuclear industry there is likely to be very long-lived and non-naturally occurring isotopes left behind. For example Zirconium-93 has a half-life of 1.53 million years.

Long term nuclear waste tends to be stored away very carefully in geologically stable sites and is likely to survive longer than any other remnant of civilization. The decay time would give you a good way to date the lost civilization.

Of course KI and KII civilizations would have better ways of dealing with radioactive waste than burying it but they would also be manufacturing isotopes to suit their needs and would leave behind isotope profiles which were not natural.

Answer 3

I am no expert, but your question got me thinking about it, so:

Cities.Even though the buildings collapse and disappear, their rubble more-or-less falls into its own footprint. So you would find a layer of the decay-materials of concrete, and steel. Sure, after millions of years this "layer" will go from being defined, to being spread through strata, but I imagine it would be detectable for a SUPER long time just by the chemical ghost.

Similarly, you are going to find fossils, which are SUPER weird. When our sewerage system gets flooded, and filled with sand, and the sand is infused with mineral-rich-water, etc., etc. - Well, it would be hard to mistake fossilized sewerage pipes as femurs.

And I could imagine there are more than a few military structures which are reinforced, and in geologically stable areas. Nuclear-waste disposal sites are built with the itnention of storing this material for ~million years and it will be identifiable for a long time after that. If some civilization discovers that, it won't be long to breach it, and inside they are going to find the "sufficiently advanced"/ magic technology.

OK, so all of that stuff is JUST what WE would leave behind if we died today. A million years is an easy timescale to still recognize who we are/were.

But for how long could a civilization remind the planet they existed? The best way would be to send something off-world, and have it come back. Here is what I might do:

Create a LOT of small containers, shaped in a way they can re-enter the atmosphere intact. Ship them off near-by to the L1 and L2 points, which are positions outside of earths orbit, but keep a fixed-poistion relative to earth so that they don't orbit us, and objects are able to stay there indefinately. Put the object nearby, but in a way in which their orbits will decay and intersect the earth's orbit eventually.

Answer 4

I don't know why nobody mentioned satellites and space debris. A piece of something that is manufactured could stay in the orbit for a long time. But, if it's too small, you need to correct the orbit. There is some discussion on SE about making satellites stay in orbit for a long time, see here the discussion on orbit correction, and here the one on making long lasting satellites.

A civilization could build something on an asteroid already orbiting the Earth and leave it there for the next civilization to discover. That will be big enough not to need orbit corrections, and if you coat part of the asteroid with glass or aluminum, it would be visible from Earth. That mysterious space object will always be a puzzle for the generation to come.

Also, if you are advanced enough, build pyramids on the Moon and put metal plates on them, so they shine. They won't be discovered easily, but they will last millions of years. Or you could go all out and sculpt the Moon like in Greg Bear's book Eon.

Answer 5

Was this past civilization human? If so, consider time capsules. People have a cute little desire to leave something behind. Before written languages were even invented, people left their hand prints and other little paintings on their cave walls, almost as if to say 'I was here!' Today, we bury time capsules all over the Earth, whether it's our own personal capsule or one organized by a school, company, or local government. If the people in your old society were advanced enough, they could have constructed some capsules that stood the test of time. How cool would it be to unbury one from a millenia ago and sift through the items of their time? It might not satisfy your scientific needs but you could do so much more building with that past society.

Answer 6

If human civilization went extinct today, future paleontologists would notice some very unusual things. Rabbits somehow migrated to Australia. Wild boar migrated to America. Raccoons somehow crossed the Atlantic to thrive in Europe. And many other species migrated from one continent to another.

All these introductions of non-native species at approximately the same time would need some explanation. (Although maybe future paleontologies would postulate an unprecedented drop in sea level rather than apes carrying animals in boats.)

Answer 7

Okay a K-I Civilisation uses all the energy available on a planet this going to leave behind a few odd traces, radio-isotopes that shouldn't be there, strata that are unusually enriched in impure silica compounds from semiconductor solar farms, lenses of oddly regular conglomerate rock from concrete cities, possibly preserving grass artifacts. K-Is don't necessarily have any space travel/industry etc... so I'm going to ignore that side of things completely but there are definite possibilities in the direction of off planet artifacts surviving as well.

A K-II Civilisation, by definition, either has a Dyson Swarm, Dyson Sphere or inhabits 10,000,000,000 worlds at K-I levels of energy density. A Dyson Swarm is composed of space habitats arranged in orbital shells that intercept all available energy from the stellar primary, you are not going to miss the remains of that in your star system if you have eyes, let alone telescopes. A Dyson Sphere well yeah enough said. A civilisation spread across that many worlds doesn't have to terraform but probably will, also you're likely to see large-scale astro-engineering in star systems with inhabited worlds, orbital shipyards, widespread asteroidal mining and "herding", pressurised habitats on small bodies like the moon.

Either level, if they use plastics, will leave traces of polymer residual chemicals in the water of any inhabited world pretty well forever after habitation. Stainless Steel, that stuff will last next to forever under oxidising or reducing regimes. They probably also use a lot of mono-crystalline diamond for construction towards the end of K-I and for most of K-II, that's either going to burn or more likely it's going to survive intact when everything else collapses around it; in fact buildings done in diamond are probably going to survive even when their foundations fail, you'll find them lying around on their sides all over the place.

Edit: I realised that there's actually a fourth option for a K-II Civilisation; the civilisation that builds 3300 odd Ringworlds; odds are that since the Ringworld is not quite stable in orbit of a natural star (allowances are made for this in The Ringworld Engineers) a galaxy peopled by a civilisation that built thousands of them and then disappeared would be home to many Ringworlds, and chunks of broken Ringworlds, spinning through interstellar space. Thought for a disaster story, what happens when a rogue Ringworld plows through an inhabited star system?

Answer 8

Jewelry is the longest lasting thing a society would make. Gold doesn't corrode and neither do gems.

Mount Rushmore was predicted to be the longest last monument mankind has made with an expected lifespan of 100,000 years.

For anything to last longer than 100,000 years, it would need to be buried

Answer 9

Here is what you need to keep your relics nice.

1. Dry. Water means life and life means chipping and gnawing away at things. Eroding things. Ice walls crushing things. When it is dry you have none of that.

2. Buried. Wind is a distant second to water as regards wearing things away but it gets there eventually. Once you are buried you are out of the wind.

3. Geologically stable. 100,000 years is not too much in geological time but you could have a lot of earthquakes. If it is flat and stable things wont get shook up or buried.

Here is a gentleman who was unearthed from underground in the Taklamakan desert, where it is dry and stable.

Maybe his lips are a little chapped, but otherwise really good looking for 3000 years old. If he can keep that well under those circumstances, other things can keep that well. Certainly structures would do fine.

Of course the best way to preserve delicate things is in amber. This lizard is 23 million years old.

Maybe there is some amber equivalent you could have encase your relics. That would be pretty cool.

Answer 10

Portions of their space structures will be intact. Plastics will have completely disintegrated, and micrometeoroids will have eroded exterior surfaces. However, given the near-vacuum (just interstellar hydrogen and solar wind), depending on their composition and location, some metal components will survive.

Objects without internal atmospheres will fare best, and those with thick hulls. For instance, I imagine a robotic mining operation burrowed into an asteroid would be highly recognizable.

Answer 11

At a million years, we start talking about geological timescales, during which mountains form and erode.

It is theoretically possible to have something -- after all, we find fossils as old as 3bn years, however whatever the surviving items might be, they should at the very least withstand different sources of erosion.

"Science clarified" lists these major sources: running water, ice, wind, near-shore waves and gravity.

So, whatever it is, it should stay the same under pulverization by pressure, when they are gobbled under some tectonic plate during mountain formation or just under tons of sediment; washed over by water and other liquids, etc.

Therefore, if the civilization is anything like ours, it is quite unlikely that anything recognizable as part of advanced civilization might remain.

Most of items we create do not withstand even 10 years of use, not to speak of abuse. Buildings will crumble in 50-100 years, unless repairs are done, Oldest ruins found are several thousand years, but from then on, we don't have seem to have anything much really in terms of structures, and all the old ruins seem to only be preserved because they have been underground, but not TOO much underground, where pressure would destroy them.

Oldest tools found are circa 2,5 million years old, but basically they are just... stones.

I liked the idea of radioactive waste deposit, but even a tectonically stable place would still have other sources of erosion, and during a million years, that plate may undergo some changes -- fractures, faults, etc. due to interaction with the other plates, so I assume during a million years all the concrete would have washed away/eroded and the deposit would have spilled.

So, looking at what we are able to find and assuming similar type of civilazation -- it would be possible to find some fossilized remains. It MIGHT be possible to find some remains of some structures which have been underground for most part of the million years, however, not if they have been moved too deep down for then they would be crushed by pressure.

Note also that while the structures would move downwards into Earth's crust, they would be not only become under more pressure, but would also prone to groundwater. Since geological processes usually take long time, they would be exposed to groundwater for a long time as well during their journey. So any sort of metallic items would probably rust/corrode away during the first ten thousand years. Any plastic would have degraded. The only things would remain would be stones.

As Terry Pratchett et al wrote in the "Science of Discworld": Deep time would wash over everything.

Answer 12

There wouldn't be any "evidence" unless you already know about that civilization. What we have are the things that "suggest" the possibility of having a civilization.

So, you define "normal" and everything that's not "normal" may suggest there's something weird going on. For example, there are a lot of fossilized clams and oysters shells on top of a mountain, it suggests long ago, there was an oysters civilization lived on top of the mountain. (or maybe oysters used to be able to fly, or the mountain used to be under the ocean..)

This method only works for past civilization that somewhat similar to the current one. Because if the past and its remnants are somehow incomprehensible then there's no way to know. Imagine a society of fire elementals swimming in magma, or society of ethereal ghosts, or mushroom...

Answer 13

If you are acquainted with the lifeboat foundation, (https://lifeboat.com/) then you might imagine that their info preserver project is successful. Which would ultimately mean that a library of information and a means of reading said library might eventually be found.

Another non natural artifact that would be obviously man made is artificially made diamonds. They are formed at high pressure using "diamond seeds" as a crystal formation template, and are easily distinguishable from diamond formed in geologic processes.