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A new species with technological potential is starting to multiply and subjugate the Earth. What resources can it harvest from human (our) cities which were destroyed 30M years ago (humans are gone now)?

Assuming some of the ruined cities still remain close to the surface and were not buried miles underneath by tectonic activity.

Split from the original question: Post-apocalypse: large mammals erased, can homo sapiens 2.0 build civilisation?

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    $\begingroup$ Nothing! Very short answer! Maybe you should shorten the time span! $\endgroup$ – Cbm.cbm Jul 5 '18 at 13:06
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    $\begingroup$ @Cbm.cbm now,now: we have recovered fossils from far greater than 3E7 years ago. Maybe some forward-thinking fellow buried books, tools, and bodies in clear acrylic blocks. (think synthetic amber) $\endgroup$ – Carl Witthoft Jul 5 '18 at 13:41
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    $\begingroup$ Twinkies, they are indestructible. $\endgroup$ – CrossRoads Jul 5 '18 at 14:10
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    $\begingroup$ What was the highest level of technology humanity had? If you have self repairing nanotechnology then probably everything $\endgroup$ – PlasmaHH Jul 5 '18 at 19:04
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    $\begingroup$ There was a TV series Life After People that speculated on the timeline of changes to the world after an apocalypse that removes all humans. $\endgroup$ – Barmar Jul 5 '18 at 21:03

22 Answers 22

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Nothing. 30 million years ago Antarctica developed its ice cap. The Alps STARTED to rise in Europe. The place now known as South America detached from Antarctica and started drifting toward North America.

The Great Pyramid of Giza was built around 4500 years ago. 30 million years ago that part of Africa didn't exist. To give extra perspective. The WHOLE region known as Italy and Greece wasn't there. There was water.

Glass needs around 4000 years to decay. You could make a glass bottle and wait for it completely decay and then make a new one seven thousand five hundred times. 7500 TIMES.

During that 4000 years there would need to be some kind of change that would stop (because slowing down would do nothing) the decay only then would it survive.

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    $\begingroup$ Comments are not for extended discussion; this conversation about animal bones, glass, and other long-surviving stuff has been moved to chat. $\endgroup$ – Monica Cellio Jul 9 '18 at 13:07
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    $\begingroup$ Please note that the claim "Glass needs around 4000 years to decay" appears to be factually incorrect. Every site I could find that was qualified to make such a claim indicated that, even in a landfill, glass would take about a million years to decay (note that this site is quoting the New Hampshire Department of Environmental Services). All of the sites claiming 4000 years were not authorities and provided no authoritative sourcing, they appear to just be copying one another. $\endgroup$ – RBarryYoung Jul 9 '18 at 17:12
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    $\begingroup$ @RBarryYoung Do possibly have a different source for "glass would take about a million years to decay"? It shows a error "Sorry, this content is not available in your region." (HTTP 401 Unavailable For Legal Reasons) if I try to access it from Berlin, Germany. $\endgroup$ – Volker Siegel Jul 10 '18 at 13:07
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    $\begingroup$ Here' a link to the actual NH DES report that this article was based on: des.nh.gov/organization/divisions/water/wmb/coastal/trash/… $\endgroup$ – RBarryYoung Jul 10 '18 at 13:44
  • $\begingroup$ What many sites have indicated is that it depends on both the type of glass and erosion. $\endgroup$ – RBarryYoung Jul 10 '18 at 13:46
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The current answers are assuming you mean from 2018 human cities. Since it will probably take some time for our ultimate demise, some technological advances could take place that would allow a species 30 million years later to detect human presence.

It is lucky that a resourceful and vain group of scientists thought of this notion in 21XX. They developed the technology for nanobots!

Each nanobot, as part of its self replicating "DNA", contains the information from the entirety of human written works. The nanobots, while ultimately being of the 'gray goo' type, reproduce very slowly. So they themselves have been migrated by geological events. They are also fragile, so extreme heat, cold, pressure render them unable to reproduce. They have thrived though in places containing unique compounds and higher than normal carbon content (i.e. cities and garbage dumps!). Of course 30 million years is a very long time, so the nanobots also reuse the corpses of non-functioning nanobots. What is left 30 million years later, after millennia of nanobot reproduction and mutation, is areas of the Earth containing pockets of nanobots, each slowly reproducing and containing encoded information from a species long gone.

ADDED BONUS PLOT

The nanobots are quite small, and basically indiscernible from "oddly colored dirt". The information encoded in them is basically irretrievable by all but the most sophisticated of beings. The nanobots, while small, do have primitive locomotion (so they can cluster near high resource locations). They find carbon-based life forms especially delicious, particularly the large spongy masses known as "brains". They are slow to reproduce (decades), so an organism may live its entire normal life with a nanobot embedded, and not notice too much.

There is an unusual effect when a nanobot or a few hundred, invade a brain. The host organism's neural activity is altered by the molecular structure of the nanobot (which contains our encoded information). It manifests itself first as hallucinations, or wild imaginative thoughts. After much training, and the nanobots thoroughly embed themselves within the host, the host is able to enter a trance-like state where vast new worlds of information are available. This leads to the host species having cultural or technological advances that leads to...

REALLY?

You may be thinking that nanobots that just happen to cause changes in brain functions is a bit far-fetched and there would be a pretty slim chance this would actually happen. Unless they were designed to do that very thing!

Neural augmentation was researched for many decades to develop the technology. It was first introduced by the clandestine, shadowy, military-industrial-complex to help create super soldiers. Of course the technology leaked into normal life. In 21XX, everyone (except poor people of course) had their memory improved by "neuraugs". They were easy to install, just take a couple of pills, and the nanobots were absorbed into the bloodstream where they were then transported to the brain.

The cutting edge research was also geared at giving new capabilities, such as:

  • The ability to use our existing vocal chords to more densely encode information, and decode this information with our existing auditory system. The neuraug soldier could communicate entire battle plans in mere seconds.

  • The ability to filter visual signals to improve vision at night or in high brightness, or in dusty environments.

  • The ability to simulate sequences of physical events in a highly-parallel manner, allowing for increased performance in hand-to-hand combat.

BUT WHY?

So 30 million years later, why would the newsapiens be interested? At first, maybe they aren't, or they misunderstand what the nanobots are. At first, they appear to make people sick, and are classified similar to viruses (neither dead or alive). However, once one segment of the newsapien population discovered how to harness some of the neural augmentation capabilities, an arms race unfolds. The newsapiens don't know how to create the neuraugs, and they cannot be grown in a lab rapidly enough to satisfy the need. So they are harvested from the few sites on the Earth where they are abundant.

OTHER CONSIDERATIONS

Since the newsapiens had a different evolutionary path from humans, their cellular makeup is similar enough for the neural encoded information to be decoded, but perhaps not as effectively and because the information was encoded from the perspective of a human host, some of the sensory signal manipulations may have novel effects that weren't originally intended. For instance, if the newsapiens have bioluminescence, or electroplaques, or echolocation, these could all be enhanced in humorous, sinister, or benign ways. (for instance, perhaps the only effect on the newsapiens is to cause a particularly attractive pattern in their bioluminescense output. So the only reason newsapiens are interested is to increase their chances at mating?)

Also, there could be mutations of the neuraugs over 30 million years, such that the neuraugs themselves have developed in to "species", each with its advantages and disadvantages.

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    $\begingroup$ Welcome to worldbuilding.SE! This was a clever first answer, especially as we, ourselves, are only just at the point technologically that we could identify the existence of a nanobot. I'll never trust the dirt in my backyard again. $\endgroup$ – JBH Jul 5 '18 at 14:08
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    $\begingroup$ Pad it out a bit and I'll buy the book. $\endgroup$ – DrMcCleod Jul 6 '18 at 9:00
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    $\begingroup$ "Since it will probably take some time for our ultimate demise..." That's wonderfully optimistic of you. Sometimes I wonder... :-) $\endgroup$ – T.J. Crowder Jul 6 '18 at 17:01
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    $\begingroup$ @T.J.Crowder The initial crash may well happen any day now, but actual extinction is a different matter. Even if every nuclear-armed nation on earth decided to coordinate their efforts to try to sterilize the planet, it'd only be about 99.999 percent or so who would die in the nuclear barrage itself. There'd still be hundreds of thousands, maybe millions, of people worldwide, isolated a few hundred here, a few thousand there, who would be royally screwed, but would take a bit longer to entirely die out. $\endgroup$ – Matthew Najmon Jul 8 '18 at 2:43
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    $\begingroup$ This was brilliant, just tell me what you do smoke? $\endgroup$ – fire in the hole Jul 9 '18 at 8:22
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Nuclear waste.

Over 90 % of naturally occurring uranium is U238, but U235 is used in nuclear reactors because this is the isotope that is fissile.

Future earthlings would find large unexplainable deposits of U235 around the sites of old nuclear reactors.

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    $\begingroup$ Not only will they find uranium more enriched (or depleted) in U235 than natural uranium, they'll find fission products which indicate the material had to have once been in a nuclear reactor. That's pretty much what happened in the Gabon. $\endgroup$ – Harper - Reinstate Monica Jul 6 '18 at 8:03
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    $\begingroup$ We don't tend to keep radioactive waste in cities... $\endgroup$ – Burgi Jul 9 '18 at 10:36
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    $\begingroup$ @Burgi actual power plants are near cities, and they generally have spent fuel on site. $\endgroup$ – Dan Neely Jul 9 '18 at 14:07
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    $\begingroup$ @Eric maybe they are better at making use of it than we are with our current technology so it would not be waste to them but something useful. $\endgroup$ – mathreadler Jul 9 '18 at 21:23
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    $\begingroup$ @Eric Nuclear waste in the US is full of useful material, primarily the Pu-239, that you can extract and reburn in a reactor. We just can't do it because of our laws. In fact, WE wanted to harvest it in the future, which was the intent of Yucca Mountain at one point - to store it away and use it in the future when able. $\endgroup$ – T James Jul 9 '18 at 23:52
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The oldest cities we have on Earth date back to some thousands year ago. Way too young to give a reliable metric on what could remain after 30 million years. But we can give it a shot based on our current knowledge on how various (broad) classes of materials would behave to ageing:

  • Oxides based materials (glass, pottery, bricks, concrete): with them we are lucky, as being already oxidized it's relatively hard to get them to a lower energetic content. The major risk comes from physical damage, reducing them to dust. I would speculate that these materials would leave various clumps of different sizes. The water soluble one would be probably gone.
  • Metals: most of them will be back to oxide state, and some of these oxides would have been solved in water and carried away from rains. Gold will stay gold, so it would be possible to find some deformed gold jewelry or tooth.
  • Carbon based materials (plastics): hic sunt leones. Plastics have the weird feature of being rather sturdy but also have an high energy content. I have the feeling there could be two paths: one path leading to simple physical degradation, with plastics being reduced in size and ending up as dust/sand, chemically almost unchanged, another path leading to the evolution of micro-organism relying on plastic as energy source. Think of it: a lot of unoxidized carbon, waiting to release its chemical energy. Over 30 million years something could evolve.
  • Organic materials (fuels, wood, etc.): unless they have found the right conditions to turn into fossils, they would have been chemically degraded by other organisms.
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    $\begingroup$ Something already evolved to eat plastic: google.nl/amp/s/theconversation.com/amp/… $\endgroup$ – Demigan Jul 5 '18 at 12:49
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    $\begingroup$ Over thirty million years the Earth is very dynamic and that's enough time is enough for a huge amount of deposition, volcanism, subduction, and erosion. Very little of the present surface of the Earth would remain at the surface. What gets eroded away is dispersed and lost, but some small fraction of what is buried might be mined. I think you're being very optimistic. $\endgroup$ – Mark Olson Jul 5 '18 at 13:30
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    $\begingroup$ I think you're severely underestimating the impact of physical damage to oxide-based materials. 30 million years is enough time to erode mountains. The only way you're going to get surviving lumps of material is if a city gets safely buried and stays that way. $\endgroup$ – Mark Jul 5 '18 at 22:21
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    $\begingroup$ The plastic (PET) eating bacterium Ideonella sakaiensis was discovered recently in a japanese garbage dump. It seems to have evolved in the last 50 years. bbc.com/news/science-environment-43783631 $\endgroup$ – Georg Patscheider Jul 6 '18 at 14:42
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    $\begingroup$ @G0BLiN, it has to be buried in the right conditions. Shore burial is about the worst possible: wave erosion, biological decomposition, corrosion, salt splitting, etc. -- you'd be lucky if anything is left after only a few thousand years. Things in valleys don't tend to get buried -- rivers make valleys deeper, not shallower, and a city is far more likely to erode than to be buried. Mountains have the same problem, with the addition of crushing by landslide. Your best bet is a desert, where dry sand can blow over the city and protect it -- but it needs to stay a desert for 30 million years. $\endgroup$ – Mark Jul 8 '18 at 18:53
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30 Million years means:

  • Multiple ice ages in temperate latitudes, reforming the ground over and over. Most cities in Europe and Russia and northern North America scraped clean by mile-high rivers of ice over and over. No traces left at all at the site, and curious jumbles of oxidized rich ores at the edges and drop-areas.

  • City ruins along slow rivers buried by sediment and then fossilized. Cities along fast rivers long eroded away, nothing but air remaining.

  • A somewhat different sea level, so most coastal city ruins either buried miles inland on the coastal plain or submerged near-offshore.

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OK, let us start with: after 30 million years, what geological activities did not to the cities, weather, erosion and vegetation did. Even plastic materials are gone, rubber, stainless steel, glass...everything. You'll be lucky to find the pattern of what used to be the cities well hidden inside the greenery.

The polar settlements have been devoured by ice. The mountain communities have long been destroyed by avalanches that inflicted the final blows to the lack of maintenance.

Any leftover artifacts save gold and crystal jewelries will be too scattered to be considered more than a lucky find, and by then any incision on the gold will have been carefully smoothed so that it will be next to impossible to consider them as ancient artifacts

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Maybe you could locate cities by the curious composition of metal oxides they might leave behind. We use a lot of quite rare elements in our every day appliances; gold, silver, titanium, tungsten, platinum, etc.; and their co-location in a single place might be hard to explain otherwise.

However, what you could easily find is Fort Knox.

Gold has the trait of being extremely inert, so much so that it's always found in its metallic form. It's just dispersed as very fine dust normally, which is what makes gold mining so hard. You have to go through tons and tons of material to accumulate meaningful amounts of gold. And this is precisely the point: We humans are by far the most effective aggregation process that gold has ever seen. We not only mine it, we also have the habit of piling huge amounts of this metal together. There is no natural process that comes close to us in this. And Fort Knox is the largest of these piles.

Because gold is so inert, the pile at Fort Knox will have a very hard time dissolving. Chemistry just won't help. Wind and water are powerless until the gold is ground back to dust. And even then, the dust won't spread far quickly. So, when all the concrete and steel of Fort Knox have withered away, nowhere to be found, and the gold has been ground back to dust and mixed with the surrounding dust, the pile of gold will remain as the most strongly enriched deposit of gold anywhere on the planet.

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Any major city that wasn't subducted by plate tectonics would at the very least be a good iron mine.

It will probably be iron oxide, but it was iron oxide when we first dug it up.

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    $\begingroup$ multi storey car parks will give you some pretty distinctive stratification too $\endgroup$ – jk. Jul 10 '18 at 12:26
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While your new species might have trouble harvesting anything from the previous human civilization, see all the other answers for details, they may still find out about humanity in a limited way.

An article from Space.com in 2011 put the survivability of the Apollo missions equipment within the time range you're talking about.

"They won't be there forever," Mark Robinson, an Arizona State University scientist and the principal investigator of LRO's camera, said in a news briefing today. "The moon is constantly bombarded with micrometeorites. These are very, very small particles that impact at very high velocities."

"In human terms, it may seem like forever, but in geologic terms, probably there will be no traces of the Apollo exploration in, let's say, ten to a hundred million years," Robinson said.

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    $\begingroup$ How is this relevant to a human city within Earth's atmosphere? $\endgroup$ – leftaroundabout Jul 5 '18 at 14:05
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    $\begingroup$ It isn't directly, but it does provide another way to appreciate the scale of time involved. The lunar landers today look practically the same as they did the day they touched down. A lander (or a car) left uncared for in the open on Earth wouldn't look as nice after 50 years. Which gives another way to think about the points in the question, when scaled up to millions of years. $\endgroup$ – Jontia Jul 5 '18 at 16:20
  • $\begingroup$ Timescales involved between Earth geologic times and Moon geologic times aren't really comparable - the two systems are so vastly different that any comparison you may make isn't useful. E.g. things on Earth have oxidation/corrosion from an atmosphere, weather effects, water+heat/cooling cycles, living organisms, etc. to deal with... while the Moon has likely vastly larger heating/cooling cycles, constant bombardment, but a notably less reactive environment to deal with (steel would likely never rust). Equipment that lasts forever on the Moon may only last years on Earth (& vice versa). $\endgroup$ – Delioth Jul 5 '18 at 21:35
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    $\begingroup$ @Delioth that's rather the point. The moon is quite good at preserving anything that can survive the vacuum, since there's no atmosphere to power erosion. Even in a much friendlier environment for long term preservation, there won't be anything left. What gives anything on earth a snowball's chance in hell? $\endgroup$ – Leliel Jul 6 '18 at 4:32
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    $\begingroup$ Probably the only evidence left after 30 million years would be a geosynchronous debris belt. $\endgroup$ – Leliel Jul 6 '18 at 4:33
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Nothing will be harvestable.

The oldest exposed ground on Earth is found in Israel's Negev Desert, at 1.8 million years. Most exposed surface lasts much less time. That means that the ground your cities stand on will have been subducted or otherwise recycled by plate tectonics. In 30 million years you will have to do some digging to find any remnants, and they will have been processed by grinding. This is why fossil finds are so rare, given the number of living organisms that have ever lived on Earth.

Your next civilization might get lucky and find a gold ring or titanium implant. But there is not enough quantity anything left to harvest.

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    $\begingroup$ I’m pretty sure plate tectonics doesn’t work that quickly. Dinosaur fossils are far more than 1.8 million years old, yet they haven’t all been melted down by plate tectonics. I interpreted the article as saying that’s the oldest uneroded piece of land on Earth, meaning that other areas may be older, but they’ve experienced at least some erosion. $\endgroup$ – J F Jul 7 '18 at 20:13
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    $\begingroup$ @JF: Yes, the land that your city was on will be gone. The surface, and anything on it, will no longer exist. It is like asking what will be salvageable from a chessboard after the table it is resting on will have been through a surface planer. $\endgroup$ – dotancohen Jul 8 '18 at 6:35
  • $\begingroup$ @dotancohen Wood would be harvestable from the chessboard, & probably at least some of the more durable raw materials would be harvestable from earth. $\endgroup$ – JustinCB Jul 10 '18 at 18:23
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Well, this looks a little like a cart before the horse situation. See, in this scenario, it seems you are wanting to set things up for story development. The thing is you need to decide what resources you want your new species to have access to. Then you need to do some research and find out how long those resources will still be available. Then set your time period since the cities were destroyed based on those criteria. Also, you need to consider whether the destruction of humanity occurred near the present or much further in the future than the current time period.

(In the next 20-50 million years possible candidate species might feasably come from present day Octopuses, most Primates, Bears, Dogs, Cats, Dolphins and Porpoises, or Elephants.)

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  • $\begingroup$ Welcome to Worldbuilding, JeremiahLadenheim, your answer provides a reasoned protocol that can be pursued by the OP to determine what needs to be salvaged from abandoned cities. Sadly, you didn't include otters, beavers, kangaroos or meerkats in your list of successor sapient species. Don't worry, that's just me. Have fun here! $\endgroup$ – a4android Jul 6 '18 at 6:35
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I am in agreement with most other answers. Anything terrestrial will be long gone. You might get exceedingly lucky and find a very rare example of something preserved, but most of what you'll get are fossils. You won't find recoverable technology on earth.

Even orbital debris will not last more than a century, according to NASA, before re-entering earth's atmosphere and burning up.

However, there's a chance you could find some technology on Mars or the Moon. 30 Million Years is a long time to try to recover anything, and if you had the technology to get there and get back you probably don't need any of it, but it's possible it could survive. I'm not so sure about equipment on Mars surviving, but the Moon has no tectonic activity, and only impacts would damage or destroy equipment. Odds are very good that something would survive (whether you could get to it, recover it, or find it, though, is another matter).

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Very few items would retain their shape and characteristics over that extreme amount of time, due to the scientific reasons proposed in many of these other answers - however, fossilized remains of the preexisting humans can be found, leading our future archaeologists to wonder if they have something to do with the millions of intelligently shaped and carved gemstones that have been found in every corner of the globe.

Surely, this primitive ancient civilization could not have had the technology to form these brilliant gemstones, and it is evidence of ancient alien intervention on our planet. Diamonds, rubies, or emeralds, all shaped and formed, how curious - they must have had a significant role in ancient society.

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  • $\begingroup$ After 30M years of erosion, not a lot would be left of them. $\endgroup$ – Tony Ennis Jul 5 '18 at 17:01
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    $\begingroup$ Not so sure diamonds would erode all that much. The diamond isn't being held exposed to a sandstorm for 30 MY - rather it comes to the surface as the embedding matrix erodes away, then, after a few centuries perhaps on an exposed surface, it drops off and is embedded again in a new sediment. Diamonds are forever, you know. $\endgroup$ – Oscar Bravo Jul 5 '18 at 18:23
  • $\begingroup$ @PhasedOut that was a great comment, gems might indeed survive almost intact... Only problem being they are rare, and our successors would need lots of luck to stumble upon old jewelery store location or some sheikh's or oligarch's cache of diamonds. Or a bank vault. $\endgroup$ – normiesc Jul 5 '18 at 18:37
  • $\begingroup$ @OscarBravo diamonds will eventually decompose because of their chemical makeup, though I don't know off the top of my head how long it would take. $\endgroup$ – JustinCB Jul 10 '18 at 18:29
  • $\begingroup$ @JustinC.B. If only there was some way of finding out... Turns out there is! bfy.tw/IzAy. I guess you're talking about spontaneous conversion to the lower-energy state of graphite? This is highly temperature-dependent. The OP doesn't say anything specific, but if we assume roughly normal conditions, similar to over the last 30 millions years, the rate of graphitisation will be vanishingly small. Survival over even billions of years might be feasible. $\endgroup$ – Oscar Bravo Jul 11 '18 at 9:37
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An answer mentioned gold. That got me to thinking about bank vaults that the gold is held in. A bank vault famously survived the Hiroshima nuclear blast just fine. Maybe 30 million years too?

I mean the bank vault is lot more durable than the building it is in as proven in Hiroshima. Having the building collapse on it will not really do anything, either. Except bury it in construction rubble that protects it from erosion.

Further, bank vaults are solid and massive so they will over time tend to sink after the the fragile ferroconcrete structures supporting them decay away.

Even for bank vaults 30 million years is probably too long to resist corrosion but it might last long enough intact to reach a stable static position on an area that is geologically stable for that long.

And the bank vault might contain things that can resist corrosion for 30 million years. Gold and platinum bars or ingots would be almost immune to corrosion and the vault would probably get them safely thru the unstable period where erosion and mechanical stress is an issue.

Of course a large enough bar of gold or platinum would be fairly resistant without the vault. And these might also survive.

Gems and jewels in bank vaults might also survive for same reasons as gold and platinum.

A large localized "deposit" of gold and platinum in large chunks of artificial shape and gems originally from widely separated geologies with cut shapes would be a clear sign of ancient civilization. I think the markings on gold or platinum might survive too.

The vault structure itself might also survive. It is durable enough to leave behind a "fossil" before it decays. And the metals and ceramics are massive enough to not get totally lost even after they decay.

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Gold, Lead, Stainless Steel, other specialised alloys devised specifically for their corrosion resistance, and possibly even Aluminium.

Not everywhere, a lot of cities will be buried by sediment or eroded away by water and/or ice scattering their material far and wide but in a few places close to the equator and far in the continental plate interiors that won't be the case, Denver for example could be surprisingly well preserved. In those places metals that either oxidise very slowly or form impermeable oxides that protect them from further atmospheric corrosion will last and last. The remains of concrete buildings could potentially keep the "city deposits" in geologically stable areas alkaline for millions of years, especially in dry climates, which will extend the life of many alloys further than usual as well.

Statues

Anything carved from physically stable rock with low chemical activity, a lot of granites fall into this category or better yet Quartzite, will last a long time if not damaged by falling debris so many pieces of statuary may be salvageable for tens, or possibly hundreds, of millions of years.

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  • $\begingroup$ A jewelry store is a good little deposit of gold. Fort Knox is better $\endgroup$ – GC_ Jul 10 '18 at 18:11
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    $\begingroup$ @GC_ If you could find Fort Knox, the Ohio River will probably have either buried it or trashed it and scattered the bullion. $\endgroup$ – Ash Jul 10 '18 at 18:14
  • $\begingroup$ @Ash Gold is pretty dense, so it's anyone's guess how far it would wash away. $\endgroup$ – JustinCB Jul 10 '18 at 18:26
  • $\begingroup$ Also, it's rare enough that they might try to mine Fort Knox even if it DOES get buried. $\endgroup$ – JustinCB Jul 10 '18 at 18:26
  • $\begingroup$ @JustinC.B. They have to know it's there, if they do then yeah it would totally be a prime mining target, as would a large area down stream. $\endgroup$ – Ash Jul 10 '18 at 18:30
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I agree with count Iblis - information.

Here is my reasoning,

Assume that humanity does not simply stub its collective toe and fall off the world in one day, that we are at least to some extent aware of our upcoming demise. it follows that we would like to gain some measure of immortality, headstones for the world as it were. we have, given our current level of technology an ability to encode vast amounts of data in small physical units, not to mention that this capacity is increasing everyday. would it not be reasonable to mass produce a smaller earth bound version of the Pioneer plaque produced by Carl Sagan? Even given the small time since then ( 45 odd years) we could dump entire libraries onto fingernail sized objects.

Encase these items in resin or amber like substance and the just drop them everywhere. I imagine ships at sea dropping them overboard.

To answer... yes a lot of these objects would be destroyed, but some would survive, especially if you consider tectonic plate activity to be analogous to the flow of a river or stream, some would collect in the eddies.

Further the same units could be launched into space, or simply dropped/shot to the moon, not useful to a planet bound civilization but maybe later on? ( 2001 anyone?)

This would be in addition to 'vaults' left by wealthy individual countries ( or people) left in geologically stable areas, or placed in bottom of ocean etc again most would not survive.. some would

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  • $\begingroup$ Welcome to Worldbuilding, Tovarisch! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jul 8 '18 at 14:09
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    $\begingroup$ I think you are answering "How could I leave something for someone coming 30 million years in the future?", while the OP is asking something else. $\endgroup$ – L.Dutch - Reinstate Monica Jul 8 '18 at 14:53
  • $\begingroup$ Very true Dutch, I was answering from the perspective of 'if we had time and had known about our demise' , other answers had already established the problems due to the massive timescale. My answer was to point out that what a future civilization may well depend on how much warning we have of our own downfall. If it was an extinction level event, then all bets are off. $\endgroup$ – Tovarisch Jul 8 '18 at 15:50
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In addition to other answers coming down to "Nothing" I would like to add something no one else did and which is crucial.

30M years will literally pulverize to dust anything that is on the surface of the Earth (to which I will include anything underground to about 10-15 meters). There will be glaciation everywhere, including down to an equatorial region (as it happened before, It will happen again) and receding glaciers will release titanic amounts of water. During specified time frame this process will repeat itself a significant number of times (maybe anything between 40 to a 100 times).

There will be other geological activity. Yellowstone supervolcano supererupted three times during last 2.1 million years. Each time it rearranged quite large chunk of North American continent (say whole Western Coast from San Franciso to Whitehorse in Yukon, and as far East as Nebraska) and indirectly impacted entire globe. Add to that the fact that there are 3 more such hotspots (Andes in South America, New Zealand, Sumatra) one can expect major rearrangements of the majority of the globe, including seabed, continents not even mentioning.

Another consequence of glaciation will be advancing and receding of oceans. No reason to believe it may not be less than already identified 150 metres each way. Since majority of humanity lives below that level, water will still accelerate the decay of civilisational remains of humanity.

This is in addition to a normal decay. It is estimated that 90% of all construction erected by humanity will not be standing 100 years after it's gone.

There is little reason to believe that what can be "harvested" after that time will be the size of a pebble. Including metals. I believe that "pulverizing power" of any glacier is greatly underestimated by mainstream science, but my opinion is it will be sufficient to crush, break or crack in any and all ways any object, including precious metal bars. So yes, Fort Knox will be gone as well. Maybe it will spark future Gold Rush of Kentucky (or whatever it will be called 30M years from now), when some lucky being will find few gold nuggets there.

Solar flares will remove majority of the space junk from around Earth (obviously not by burning, but definitely it will impact orbital movement time and time again), magnetic pole reversal will further impact any and all geostationary (more or less) objects in near space.

Maybe, just maybe, there will be something to salvage from eventual Moon base on the bright side of the Moon, but it all depends...

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Fossils.

We routinely find valuable evidence of plants and animals from 30 million years ago, a period of time called the Oligocene.

Fossils come in many forms:

  • Cast/mold fossils are created when a buried object dissolves away and the mold is later filled with other minerals. Most of the answers here talk about how our metals and even plastics will eventually decay, but even after that happens, they will create casts. Imagine finding a keyboard-shaped fossil!
  • Trace fossils are things like footprints or feces. If a dinosaur footprint from over 64 million years ago can still be found, surely the much more prominent marks created by our automobiles and construction projects will have a reasonable chance of surviving.
  • Resin fossils are made from naturally occurring polymers like sap. Insects and other objects get trapped in the polymers, which eventually harden into amber. It's reasonable to think that out of the many types of polymers we have invented that some of them may behave in a similar way.
  • Wood fossils can be found in petrified form, where minerals have replaced the original tissues. It's reasonable to think that the wood that makes up our structures could also be petrified and even remain positioned like they are today. That would be a fascinating archaeological discovery.
  • Chemical fossils are concentrations of molecules which indicate life. Our landfills will almost certainly produce strange chemical signatures relative to the surrounding rock. As other posters have mentioned, our radioactive waste is an obvious case of this.

How are fossils valuable resources?

Chemical fossils may obviously be useful to alien visitors. The geological processes by which they are processed may put them in exotic forms. Think about how we use oil! (While this timescale is not likely to produce oil, there may be something of value that our geologically strange materials produce in large quantities).

However, I think the paleontological value of alien fossils may be a thing that intelligent species put great value upon. I can imagine a thriving galactic market for unique and rare traces of life.

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to strike off on a tangent from the other answers, I can think of two ways your new intelligent species might find some trace of our cities. They are oddly related...

  1. Genetically engineered or just heavily domesticated species. What if humans geneered trees that grew into houses - and those escaped into the wild after the Fall of Man and some pockets survived, like California Redwoods. Or somebody geneered a solar-powered paver that reproduces very slowly - when walked on by living feet. We can't do these things NOW, but in 300 years with a lifespan of 600 years, today's trippy sci-fi becomes that era's Home Depot special. What if before we fell, some enterprising pet store geneered a talking meerkat and they were a big hit? Maybe your new rulers of Earth got a nudge toward civilization from us, all those Mya.

  2. Artificial 'life' - robots and cybernetic systems. If we survive the next couple of centuries, we might thoroughly automate the infrastructure of civilization, from mining and recycling through manufacturing, distribution, and maintenance. And we might be wise enough to make our intelligent ecosystem adaptable but highly resistant to mutation i.e. lots of bits in the error-correcting codes. Maybe sentient - but maybe more like an ant colony. The result might be a humanless city, or city-like thing, that evolves VERY slowly, but deals well with geologic-scale change - digging itself out of the ice, migrating slowly after the retreating coastlines to maintain some long-ago ideal 'beach access'. I don't know what it would be like - but "Solaris" might be a better inspiration than "I, Robot".

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    $\begingroup$ 30m years is a very long time. Most changes humans make to domesticated creatures and plants don't make them more viable in the wild, but rather on the farm. It's doubtful those changes would remain viable in the wild. $\endgroup$ – Marshall Tigerus Jul 9 '18 at 15:35
  • $\begingroup$ @MarshallTigerus sure, but as you say, "most", "doubtful", etc. - this is fiction, so we are not obligated to select only high probability events. And the human genetic interventions of the next two centuries will probably not be like the past. Scientists are well aware of what you say - there will be quite some fame for the first team that engineers an organism that is highly viable in the wild using some design innovation never discovered by natural evolution. probably such a thing won't ever get loose ;-) $\endgroup$ – Spike0xff Jul 12 '18 at 16:44
  • $\begingroup$ I don't disagree that it's fiction, so anything is possible, but that "rule" basically invalidates any questions and answers here, since it's fiction, anything is possible. From a genetics standpoint, it would be incredibly unlikely for even the best genetically engineered species to thrive and retain their genetic alterations. Given how "weird" many forms of natural life are (looking at you, platypus) it's very likely a followup civilization would never realize an organism had been artificially engineered. $\endgroup$ – Marshall Tigerus Jul 12 '18 at 16:56
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Souls

As previous answers have mentioned very little physical or technological remains will be left.

But, depending on the genre you're creating for you could harvest some immortal part of the humans who once lived in these cities?

As the science of the soul isn't exactly understood this gives you terrific flexibility around how this energy/consciousness/life force is harvested. It could be gathered by robots, wizards, or aliens.

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Information. The new developing civilization can benefit from our scientific knowledge if they can decipher and read our sources.

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The other answers are too pessimistic.

We currently power our cars and planes from the remains of dinosaur civilisation, 65M years ago (yes, and mostly algae and plankton).

25M humans in one city is a lot of oil, gas or coal in 20M years time.

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    $\begingroup$ Most Coal comes from the Carboniferous, over 300Ma ago. It will not be replaced in 20M or likely ever. Oil comes not from "dinosaur civilisation" but from marine organisms. It will be replaced over time, but we have plundered the resources created over the last half billion years or so, and we have preferentially drained those reserves most convenient to access. There will be no useful replenishment in a mere 20Ma. $\endgroup$ – Jack Aidley Jul 6 '18 at 11:34
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    $\begingroup$ @JackAidley, peat beds are a precursor to coal deposits. It's not clear how long it will take, but some of the peat moss growing today will eventually turn into lignite coal. $\endgroup$ – Mark Jul 6 '18 at 22:16
  • $\begingroup$ @Mark: to some extent, yes, but the large deposits created in the Carboniferous? They're likely never coming back. The Biosphere no longer behaves like that. $\endgroup$ – Jack Aidley Jul 7 '18 at 7:19
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    $\begingroup$ Maybe the next civilization will notice the unusual lack of oil. $\endgroup$ – user9981 Jul 7 '18 at 15:31
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    $\begingroup$ @JF By reaching a point where they understand the geological processes and precursors well enough to notice discrepencies between the Carboniferous algae population figures indicated by the amount of oil they detect, and the algae population figures indicated by other pieces of geological evidence. $\endgroup$ – Matthew Najmon Jul 8 '18 at 2:59

protected by Separatrix Jul 10 '18 at 11:46

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