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Ok, quick overview of the situation: An alien planet, reaches basically the same stage we are at now in regards to fossil fuel reserves and environmental loss, a major war breaks out, and after both sides suffer basically equally, they decide to merge forces and revert their damage on the planet. buildings are ripped down, power-plants dismantled, holes filled, forests replanted (relatively) and any damage they can reasonably reduce is acted upon. Most of the population then leaves the planet, now with plans of colonizing a new planet, with a sustainable economy. The odd 2-3 billion or so remaining population then create a great city, far underground, to let the planet heal and periodically (as in every few years) monitor its progress. Thousands of years pass, forests and other environments are basically completely healthy, and humans orphaned of their planet colonize this alien one.

After that wall of exposition, here's the problem. In this story, the humans only realize the planet is inhabited about three generations into colonization, and this is only because the natives noticed them. How, after mining and building their own civilization, do these humans not notice the scars created from mining and past cities?

Things to note:

  • The humans came on a makeshift craft, a last minute escape if you will, the economy quickly crashed within the first two generations due to competition with agriculture and alien weeds/pests
  • Pests are beginning to be controlled at present, as well as development of alien crops, so the economy is slowly rebuilding, but the civ is basically scattered towns and villages with one main capital city. This means the civ is actually quite small, and most of the planet is unknown exploration-wise.
  • Humans don't really have resources to expend on expansion or exploration (someone already mined most of it) and instead focus on military, agriculture and infrastructure. This means any satellites are cheap and specifically for communication, not thermal imaging or the like.
  • The mothership isn't an option, it is still in orbit and has been completely stripped of parts and energy reserves.
  • The city being hidden isn't the focus, this is specifically about the mining and infrastructure scars, however feel free to add such information to your answer, as long as it's a side note. I might make a new question for that topic in the future.
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    $\begingroup$ If the humans aren't expecting aliens, they will likely look right past or rationalize the subtle hints, especially if you've had a few thousand years of erosion and natural intrusion. Some crevasses in the hills? Why, that's the remains of an ancient rockslide. A water-filled hole? That's a crater from a meteorite that became a lake. Your background also suggests that the humans lack a good world map: it's entirely plausible that they happened to land on a piece of the planet that simply didn't have any serious mines. I'd be more worried about the remains of the ancient alien cities. $\endgroup$ – Palarran Nov 11 '16 at 6:02
  • $\begingroup$ About the "majority of population leaves the planet" part -- an operation on this scale would be very, very costly to the environment. Consider letting them stay so as not to do more damage $\endgroup$ – Zxyrra Nov 12 '16 at 0:16
  • $\begingroup$ @Zxyrra I'm sure you're correct, but how exactly would it deal this damage? $\endgroup$ – XenoDwarf Nov 12 '16 at 4:31
  • $\begingroup$ @XenoDwarf Building enough ships to move billions of people would mean mining and refining extreme amounts of metal and other ship parts, and siphoning the planet's fuel sources dry. With additional food requirements to support the populations leaving (food for billions for weeks), and resources for each colony built (metal, generators, computers, seeds etc) x 1,000,000++ colonies $\endgroup$ – Zxyrra Nov 12 '16 at 4:33
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    $\begingroup$ @XenoDwarf Alternatively this civilization could be spacefaring and personal ships are the norm - picture Star Wars or Futurama or something - so they have the means to exit with some handwavium fuel or propulsion, and they take their in-home hydroponics with them or something $\endgroup$ – Zxyrra Nov 12 '16 at 4:41
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1. Why did no one notice any of the "scars" when they first arrived in their mother ship?

This is already kind of tricky I think. Your settlers were on a desperate, last-hope kind of mission to survive and given the ratio of inhabitable planets to the size of space... Well, finding this planet is like winning the lottery time 1000. So, when they arrived, they scanned the planet for atmosphere, climate, fresh-water supply, flora and fauna to gauge a general human-survivability, easy to miss any kind of shadowmarks, but they would also need to do different kinds of visual scans to determine a landing place.

I guess a spaceship on a long and desperate journey through the universe could be kind of bruised, with all kinds of systems destroyed or malfunctioning, including life-support, which would make a rushed departure without proper checks plausible... Some kind of "Either we make it to the ground on this planet, or we die anyways" situation, which would also make people blind to any clues since their whole focus lies on survival.

2. Why did no one notice once on ground?

Well, I guess your settlers just hit a once remote, almost uninhabited spot on the planet - think a former dessert or jungle - which became a good settling point after the climate changes the locals caused (I guess they would have had a similar fosile-energy focused technology as we do). If you would transform the Sahara to habitable land, you would have a hard time finding any traces of a former civilization there, since there already are almost none and erosion in desserts or plants in jungle areas are very good in destroying structures.

Now, after a few years, someone should be like "guys, this is a nice spot, lets see whats over there, a couple hundred miles west", because human nature is that of an explorer... We could have the settlers place be something like australia, with a huge dessert and cities only on the coast line. After the poles melted (yeah, that would have happened...) and the sealevel rise, the cities are now gone, some kind of golf-like stream turned and the once uninhabitable dessert turned into a nice garden. That would give your people a small continent with almost no signs of civilization to explore.

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  • $\begingroup$ This is an interesting idea (though being Australian I can say there are inland cities) the isolation might provide problems with some of the story I'm planning, but the desert idea is gold. $\endgroup$ – XenoDwarf Nov 11 '16 at 11:56
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    $\begingroup$ I think some inland cities are no problem, you can just have them being concentrated on places that already had water supply (rivers like the nil, okavango, or simple oasis) which were the first to explode with flora after the climate change, doing a good job in hiding or destroying the remains. $\endgroup$ – Ninigi Nov 11 '16 at 12:15
  • $\begingroup$ And yeah, I was thinking I might be feeding into stereotypes with australia a little :P $\endgroup$ – Ninigi Nov 11 '16 at 12:15
  • $\begingroup$ I could make this work without island isolation, if there was a desert/plains area surrounded by forest and/or mountains, then simply adding discouragement would prevent exploration, e.g. toxic jungle. The landing shuttles or whatever descends from the mothership will be forced to land in the clearing due to dense jungle, and when they discover the toxic nature they might be forced to settle. $\endgroup$ – XenoDwarf Nov 11 '16 at 12:25
  • $\begingroup$ That actually makes a lot of sense. Take North America, rocky mountains, they are the reason for the whole nevada-dessert mess... You have the sea at one side and mountains at the other and thats already a formular for dessert and volatile climate. And a forrest replacing just a small patch of former dessert could have a drastic effect on which places are inhabitable and whicch are not. $\endgroup$ – Ninigi Nov 11 '16 at 12:42
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There is no need to hide the signs of previous mining, industrial, urban or technological activity on the planet. After thousands of years most of the obvious signs would have been overgrown or degraded by natural forces. The surface will look more like that of a previously inhabited planet, but not a currently inhabited one. Basically there's no reason to suspect the planet is occupied by a civilization of sapient creatures.

The lack of any native inhabitants by itself will suggest the local sapients have abandoned their planet. After all, that's what they mostly did. Apart from those living deep underground in their great subplanetary city.

With this set of circumstances, the human colonists will assume that the planet was inhabited in the past. Wars and runaway consumption ruined the planet. The native sapients moved to another planet elsewhere. Also, this is comfortable story that fits in with their belief they had arrived on a planet suitable for colonization. People do tend to belief what they want to believe.

So when native aliens emerged from deep below this would have been a nasty shock.

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  • $\begingroup$ not what I was planning, but just as interesting, I might talk it over with the other OC (this is a joint creation). $\endgroup$ – XenoDwarf Nov 11 '16 at 11:59
  • $\begingroup$ Glad to help a fellow Aussie. Archaeologists use aerial photogrammetry and remote sensing to locate ancient ruins and the remains of previous civilisations. Interstellar colonists will be better doing at this. What can't be concealed, can be overlooked by our psychological blind spots. $\endgroup$ – a4android Nov 11 '16 at 12:10
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If the situation on the alien world is comparable to present-day Earth, it can't be hidden.

  • Consider cropmarks and shadow marks. Of course they show best on planted fields, but modern buildings are so much bigger. They will show on meadows.
  • If they fought a war, there is the issue of unexploded ordnance.
  • Refilled mine shafts will last for millions of years.
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They were aliens. A group of them might easily identify traces as proof of the past but the humans don't recognize what they are looking at.

Maybe they mine using rivers, or creating fault lines, or digging up from below. Their cities could have avoided straight or even continuous lines making identifying marks look random leading to the conclusion they aren't marks.

The stewards might also be good at their jobs. After recycling almost everything to build their massive post war projects, and burying the remaining evidence under 10's of meters of soil they triggering volcanoes to obliterate even the toughest stains.

Much of this rests on the assumption that a billion person city far underground implies these people are really really good at manipulating earth and rock.

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mining could easily be covered by reclamation laws and erosion, and not everyone is going to realize an old mine hole is actually a mine and not a natural formation. holes turn into lakes fairly quickly, and the longer they have been left alone the less obvious they will be. Ones in very dry areas will stick around the longest, of course maybe the humans did not go to the deserts.

cities are a lot harder, they tend to stick out.

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