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I am making a world with a gigantic structure the surface size of a smaller continent. This structure is suspended about a hundred meters above ground, and before the construction the area below was covered with meadows and forest, with animals living in them. My question is, what would the consequences, short- and long term(like 50 000-100 000 years), of this giant structure be for the vegetation and wildlife below it?

Edit: The structure is supposed to block sunlight from reaching the ground, so I'm asking how that would affect the life below, in small and large time scales. Another thing I'm wondering about is, what life could live below the structure, and what conditions, given that sunlight or similar is not available, would be necessary to allow that life to live there?

Edit: Suspended may not have been the correct word, as it's actually held up by lots and lots of large pillars.

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    $\begingroup$ Well, for starters, you really don't want it to fall. $\endgroup$ – user14624 Jul 10 '16 at 3:29
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    $\begingroup$ What live in caves? Dripping water, bugs, bats, blind fish, fungi... $\endgroup$ – WernerCD Jul 10 '16 at 4:48
  • $\begingroup$ An advanced civilization could presumably harvest fungi and bacteria which grow by means other than photosynthesis. More advanced civilizations could use geothermal energy to produce as much light as is needed to grow most crops. Also, any civilization advanced enough to build this would probably be advanced enough to put lights and irrigation on the bottom of this to minimize the impact below. $\endgroup$ – krowe Jul 10 '16 at 5:03
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    $\begingroup$ You should also consider what happens on the edges of the structure, if it is shaped in such a way that waster flows off of it, for example, lots and lots of water would cascade down at the edges, turning the region into swamps or lakes $\endgroup$ – Annonymus Jul 10 '16 at 7:38
  • $\begingroup$ Does the planet in question have plate tectonics? $\endgroup$ – Patricia Shanahan Jul 10 '16 at 13:44
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If it blocks sunlight, no plants will grow below it, and everything that can't move out of the area will die. Plants will die in place; animals more than a few tens of miles from the edge probably won't find it before they die.

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  • $\begingroup$ I'm not sure if it does, possibly not during dawn and dusk, when the light hits from a sharper angle. $\endgroup$ – ArborianSerpent Jul 9 '16 at 21:28
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    $\begingroup$ It's continent-sized. A hundred metres of elevation is nothing on this scale. If your world is round, no sunlight at all will get more than about 70 miles in, due to the curvature of the ground and the structure above it. If the world is flat, a very little light will get in to the centre, but very much less than from an open sky, by a factor of thousands. If you want to avoid killing everything below the structure, it has to have large holes to let in sunlight and rain, or you need a completely different ecology. $\endgroup$ – John Dallman Jul 9 '16 at 21:37
  • $\begingroup$ No, I don't want the sunlight in, an organism that really doesn't like sunlight lives under it, they're advanced so they have import and the like to get food. $\endgroup$ – ArborianSerpent Jul 9 '16 at 21:41
  • $\begingroup$ Probably easiest to grow food on board the structure, and exchange it for minerals with the people living on the ground. There is presumably nothing to mine on the structure? $\endgroup$ – John Dallman Jul 9 '16 at 22:19
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    $\begingroup$ @ArborianSerpent If there's plenty to scavenge, don't expect it to last 50,000-100,000 years :) One reason ancient ruins look like ancient ruins - all shambles and broken pieces - is often that that they've been picked apart. $\endgroup$ – Flambino Jul 10 '16 at 3:14
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Given that the question has reality-check tag:

First the answer to the question. The effects are quite exactly that what Thucydides have mentioned. Every creature, animal or plant needs energy to survive, without sun zero energy at disposal => the area under the structure is completely dead.

Second due to the Venturi effect the wind below the structure will reach very high speeds which causes erosion . Normally friction of air should slow down wind, but at 100 m height the effects even for the size of a continent should be negligible. So frugal earth will be deported, like the Dust Bowl in the 1920s. ADDITION: After Patricia's justified objection: Pillar structures will slow down the wind for a structure with continental scale.

Third everything depends on how the area under the structure is: Does it normally produce more water (condensation under the structure, rivers and aquifers) than it loses or does it lose more water (dry terrain, no rain) ? In one case it will be swampy area with fungi which produce fermentation gas and the fungi will invade the structure and the pillars from below or it will be a extremely dry, lifeless area with wide trenches of broken earth.

Third the material cannot stay permanently because we have geological timescales, the area will sooner or later begin fold up or down so that the pillars (see below) cannot balance the area above anymore.

Further questions: 1. What is the purpose of the construction ?

You really may ask why the structure was built in the first place. Even a megalomaniac dictator want something with his big projects even it is only a 200 m high image in the mountain. Why it is necessary to build something like that, especially with a very resistant alien metal ?

ADDITION: If it was something to protect the alien life, why it is not possible that the material is partially transparent and filters only the damaging wavelengths ? Normal glass blocks almost completely UV rays, plants are absorbing mainly red and blue light (that is the reason plants are looking green, it is the strongest unabsorbed wavelength). They can also grow under near infrared which is almost invisible to us. We have the technology now to filter out undesired wavelengths, so aliens will not have a problem to do that. For better realism: Let the structure have big holes and definitely erosion marks. Even very hard materials will have worn down after 100 000 years.

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  • $\begingroup$ It was made to house a species of sapient life on the planet, accounting for exponential growth for some generations, why it's in the air, I'm not sure, but it's possible that the structure initially provided some sort of means by which farming was possible below it without sunlight, as these creatures will boil alive when directly exposed to high energy light. The structure is no longer operational, so it provides none of that farming assistance anymore. $\endgroup$ – ArborianSerpent Jul 10 '16 at 2:11
  • $\begingroup$ I have not yet decided on what kind of geology and such the area below has, but I'll keep your answer in mind when doing so. The thing about the earth shifting below it is something I didn't consider, so I'll see how that affects the purpose I have for it. $\endgroup$ – ArborianSerpent Jul 10 '16 at 2:13
  • $\begingroup$ @ArborianSerpent Any chance you could build some light ducts into the upper floor? $\endgroup$ – John Dvorak Jul 10 '16 at 8:40
  • $\begingroup$ Was this sapient species alien to the planet? If so, the farming they did probably involved alien plants and animals, so the whole ecology of the thing is unknown for us, and there is a whole unknown natural history of the understructure country that we also ignore. If not, how did they survive on the planet before building the structure? Also, what is the shape of the structure? Is it a gigantic circle, square, whatever? If it is circular, I would expect a series of concentric circles transitioning from normal life in the outside to complete lifelessness in the centre. $\endgroup$ – Luís Henrique Jul 10 '16 at 12:55
  • $\begingroup$ In projecting strong winds, were you considering all the pillars holding it up? I was thinking that turbulence around the pillars would cause a drop in wind speed a short distance in from the edge, with deposition of dust and water. $\endgroup$ – Patricia Shanahan Jul 10 '16 at 14:52
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The direct effects will be to kill all plant life under most of the structure (plants will still get some sunlight around the edges), and thus the animal life as well.

Secondary effects will be the regions towards the centre will be cold, dark and wet, which means that eventually the regions under the structure will be filed with fungi. As well, since there will be a great deal of water accumulating under the structure, the water table underneath will rise and eventually springs will form near the edges as the high water table seeks escape (much will depend on the underlying geology, however).

What will certainly change the picture is how the structure is suspended. It is implied that the structure is suspended in mid air by some sort of anti gravity. Since negating gravity (according to modern physics) means essentially countering the warping of gravity due to the planet below, there will be a massive energy gradient (If Earth is "pushing down" on space-time with 5.972 × 1024 kg of force, you need to exert an "upwards" push of equal magnitude to neutralize this.

enter image description here

How this is done will affect a lot of the scenario, technological fixes need to deal with fuelling or otherwise powering the structure, and dealing with waste heat. IF the anti gravity is inefficient, the structure could be glowing red hot in the sky, with obvious changes to what is happening both above and below the structure.

Of course the simple solution is to simply build a series of tall pillars and plate the space between with sheet metal, much like some sort of monster bridge. In that case the end result will be the area under the structure will be contaminated with rust and pieces of metal as the structure gradually collapses.

enter image description here

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  • $\begingroup$ Two things, firstly, why would water accumulate under the structure, and two, its made of an alien material which is extremely long-lasting, like billions of years long lasting with some upkeep. $\endgroup$ – ArborianSerpent Jul 9 '16 at 22:52
  • $\begingroup$ Condensation. The area is without sunlight so will become relatively cold. Since you failed to mention the other part in the question, I went with the easiest assumption that it is made of metal. $\endgroup$ – Thucydides Jul 9 '16 at 23:54
  • $\begingroup$ I think water would also accumulate there due to the fact that sunlight cannot evaporate it. $\endgroup$ – Aarthew III Jul 10 '16 at 0:07
  • $\begingroup$ Oh right. I didn't think it would play into the answer that much. $\endgroup$ – ArborianSerpent Jul 10 '16 at 2:05
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Not only would the plants and animals above ground would die, most likely the animals below ground would die as well. The structure would prevent rain from falling to the ground. Even extremophiles, animals that gain no energy from the sun, would die as there would be no new rainwater to seep into the ground. Maybe if there was a vast underground river system that flowed from an uncovered area... But even the Amazon and Nile only supply life to a small part of a continent.

And speaking of rain, the structure would have to be insanely strong. There would be vast amounts of rain and snow collecting on it. The weight would be enormous.

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  • $\begingroup$ Not snow, the planet has a warmer climate than earth, and the water could slide of if it's convex, right? I suppose the ground in the very middle would be very dead. If there isn't any water, would the dead biomass there just stick around without ever being decomposed by bacteria? $\endgroup$ – ArborianSerpent Jul 10 '16 at 15:02
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If the continent size structure is held up by anti gravity instead of magic, it can be equipped with fusion powered sun lamps on the bottom side to light the ground below. If it can be held up by magic, it could have magic sunlamps on the bottom side, if that is what the writer desires.

And maybe it can condense water vapor out of the atmosphere and water the ground below as well as using some of that water for itself.

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    $\begingroup$ This is not answering what the consequences are, just how to avoid them. $\endgroup$ – Hohmannfan Jul 9 '16 at 21:51
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    $\begingroup$ That's not really what I'm askin though, I was wondering how the structure, as it is, would affect the life below. $\endgroup$ – ArborianSerpent Jul 9 '16 at 22:00
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So, Sentient Species moved to Host Planet, and built a Structure, made of Solid Unobtainium, in order to make livable space for them. The basic purpose of the Structure was to provide shade for Sentient Species, who would be unable to live under the direct light of the local sun. Later, Sentient Species went away or died out, and only the Structure remained.

In this case, it seems to me that the purpose of the Structure was to provide an ecosystem similar to that on the Home Planet of Sentient Species. So life would have to be possible Under the Structure, though it could be the case that it would be impossible for Host Planet life forms to live there. The question then would be, once the Structure is abandoned and no longer maintained, would exo-life from Home Planet be able to thrive, or would it die out, leaving a dead, uninhabitable part of Host Planet - Underthestructuristan?

Roughly, if the ecosystem under the Structure needed conscious intervention from Sentient Species, then it will eventually die out. Otherwise, it will probably last as long as the Structure is able to resist erosion and biodegradation.

Sentient Species, and whatever species it cultivated under the Structure would, however, be blind. Unless, of course, their problem wasn't exactly sunlight, but:

a) a specific wavelenght, for some reason absent in Home Planet but present in Host Planet; or

b) intensity, if they can resist light, just not as much light as that provided by Host Planet's sun.

In these cases, Solid Unobtainium doesn't need to be totally opaque; it could be translucid or event totally transparent except for the forbidden wavelenght. Then maybe Sentient Species can "see", but not the same wavelenghts that are visible for us (and/or for Host Planet animal life), and maybe it is exactly the visible spectrum of Host Planet that is deatly for them. Ie, they need light, just not the same light as Host Planet life forms need. If this is reciprocal, then neither life under the Structure can spread out of its limits, nor life from the rest of the planet can invade Underthestructuristan, not at least before a long and complex process of evolution. If it is unilateral, then life from the outside will creep in, and eventually replace the exo-ecosystem.

In case the problem is intensity, moreover, they could use a different solution: settling by the polar region of Host Planet, where light intensity would be much lower than closer to Equator.

Also, besides its optical qualities, Solid Unobtainium would be different concerning whether it was conceived in order to be non-biodegradable under Home Planet conditions (in which case it could be vulnerable to Host Planet's microbiosphere), or, on the contrary, invented to be resistant to Host Planet's environment, while it would have to be maintained to resist Home Planet microlife.

But, beneath all this, lies the issue: you have had us agreed to suspend our disbelief concerning the nature of this strange material that was used to build the Structure. If you now try to give us too much scientific or "scientific-ese" explanations for what happens under the Structure after its builders are gone, you risk have us requestioning the whole nature of Solid Unobtainium.

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  • $\begingroup$ First, to clear up a few things, the species who built the structure are not the species that the structure was intended for, it was intended for a species of the Host Planet that previously lived in a low light environment, now unavailable to them. Now, if the structure was opaque, would that leave the area below the structure absolutely dark, no light at all, or with very dim conditions? The weakness of the species under the structure is high energy light, so low energy light is fine, they can survive that. If complete darkness, the structure could let a dim light bulb's kind of light throug $\endgroup$ – ArborianSerpent Jul 10 '16 at 20:28
  • $\begingroup$ I see. The problem would be just lack of maintenance then? Is the protected species sentient too? Anyway, I would go with translucid Solid Unobtainium, or perhaps even better, a structure with several small holes that allows both light and rain to come in. $\endgroup$ – Luís Henrique Jul 10 '16 at 22:03
  • $\begingroup$ Yes it is, and fairly far into their development, though I doubt they do much maintenance, as it is a gigantic structure and they only have a few and far between cities in the area. I think translucid Solid Unobtanium fits my needs better. As for the water maybe there are some channels, or it might just flow to the edges, although it would make sense for there to be channels if the structure once supported agriculture of some kind below, which I think is the reason it was built so high above the ground to begin with. $\endgroup$ – ArborianSerpent Jul 10 '16 at 22:53
  • $\begingroup$ Glad it helped! $\endgroup$ – Luís Henrique Jul 10 '16 at 22:58
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Soil tends to act as liquid over millenia. A continental sized mass, standing on pillars, will cause the soil under it to subside until the volume of soil displaced equals the mass of the structure, pillars and all.

Assuming this does not cause the upper structure to collapse, the surrounding water will eventually start to flow into the depression. Since the depression will be a bowl extending past the limits of the structure above, you effectively have an underground sea.

The animals and plant life originally underneath will have died or fled, barring some survivor species like rats and cockroaches, and when the water starts flowing in, it will bring in fish, insects and seeds/spores and algae. Some of these will find an evolutionary niche, especially around the edges, where the sun actually shines. Others will evolve into deep sea bottom dwellers like those found at the bottom of the oceans. In the closed environment, there will initially be no predators, other than what the water brings in, so the species that do arrive have the opportunity to adapt to their new home. Warm blooded animals will be at a disadvantage, unless there is some heating mechanism like hot springs nearby.Most animals near the centre will probably be cold blooded, most likely fish and amphibians.

Now, given that the salinity of the sea is due to evaporation, and there is insufficient sunlight over the majority of the surface, this will presumably be a freshwater sea. However, this poses a question of: if water flows in at a sufficient rate to create the sea, where does the excess go? This implies underground rivers connecting to other water bodies, through which new species can enter. It also implies the existence of currents within the sea through which biomass from the sunny edges can enter the darker parts of the sea, on which the deeper water denizens can feed. I would expect normal freshwater species around the sunlit edges; ambush predators in the gloomier parts, which hide in the darker water but approach the sunnier parts to feed; and highly evolved large predators in the darkest parts, which feed on the ambush predators which venture in too deep.

The 100m gap may also host some flying species, like bats, which feed on amphibians or fish that come to the surface, e.g., near the bases of the pillars. Given the probable amount of food available, these should be rare.

Edit: Mountains on the (former) surface would act as islands, providing potential habitats, assuming they weren't levelled during construction

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  • $\begingroup$ I'm not sure how the water would flow in this scenario, are you saying that the pillars would displace the dirt and then create a pond around them or something? I'm not sure how this translate into underground sea. Could you explain further? $\endgroup$ – ArborianSerpent Jul 12 '16 at 21:14
  • $\begingroup$ @ArborianSerpent: Heavy objects gradually sink into the earth. This is observable even in the short term, it's called subsidence. In this case, given the "continent-sized" scale, to prevent subsidence at variable rates, you'd probably have to create a flat reinforced platform as a foundation, before you build. This platform would displace soil, like a boat displaces water. Assuming this is completely flat, as in, the edges are not raised, compared to the base, you would expect that soil would eventually flow back in. Think putting a plate in water, then adding a weight in the middle. $\endgroup$ – nzaman Jul 13 '16 at 7:36
  • $\begingroup$ @ArborianSerpent: The plate pushes water out from under it, but the water flows in over the edge. Given the continent sized structure, I am assuming the presence of lakes and rivers, which will fill in the gaps well before the soil does. This assumes that the sources of these water bodies are unaffected and continues to supply them, or rather the depression. The sea is assumed to be sufficiently distant that it won't simply flood the gap, creating a lagoon/bay. Rainfall on the upper structure also needs to be taken into account, as the structure is a fairly respectable mountain range. $\endgroup$ – nzaman Jul 13 '16 at 7:42
  • $\begingroup$ Right, so the seas wouldn't be underwater then, they'd form on top of a foundation platform, right? $\endgroup$ – ArborianSerpent Jul 13 '16 at 15:48
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    $\begingroup$ @ArborianSerpent: I think I understand the question now. Will it be below sea level? Probably, considering the weight of the structure above. But yes, the sea would form on top of the foundations, underneath the structure. I said, "you effectively have an underground sea" in my answer, which was probably the root of the confusion. We treat it as an "underground sea", because the structure above prevents sunlight and fresh air from reaching the sea surface, meaning living organisms have to adapt the same way they'd have to in a naturally occurring cave system of water. Just on a bigger scale. $\endgroup$ – nzaman Jul 14 '16 at 12:04

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