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So Earth has just suffered a horrible catastrophe and most of civilization is back in a Middle Ages level of development. A few people live on other planets relatively unscathed, with highly advanced technology (But nothing inconceivable. No teleporters, no going faster than the speed of light, and computing never got past basic optical processors, so it's only a few orders of magnitude faster than the current fastest supercomputer.) They have decided not to try to let people on Earth gain technology because they are afraid that they will lack the ability to use it right.

Unfortunately, the climate is completely unsustainable and requires maintenance to prevent it from spinning completely out of control. Like CO2 levels at brinkpoint, weird changes in climate, etc. Their solution is to put a refinery on the planet where they use large amounts of energy (they have large amounts of energy) to collect CO2, CH4, and other atmospheric pollutants and re-release harmless O2 and H2O.

However, I can't think of a way to put this on Earth's surface without allowing the inhabitants to know. I would like not to have to set aside an entire continent.

My first idea was radiation (covering only the boundary / the plant is automated and can handle radiation), but that has a tendency to spread. I was wondering if there was any way to create a barrier that would prevent middle age-level technologically advanced people from entering. (I'm thinking they have no gunpowder, no animals (all killed off in disaster), and little wood because of the climate's effect on forests. So not that much of a threat, just don't want them snooping around and getting advanced technology.)

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  • $\begingroup$ Well, I don't know how much militarizes have to deal with curious middle ages-era armies. However, if this answer is too broad, I apologize. $\endgroup$ – k_g Mar 12 '15 at 0:22
  • $\begingroup$ A hole in the mountain and a big hidden iron door would stop most of them for some time (think Norad or any other underground military base). Especially since there is no wood to use for breaking the mountain to pieces. $\endgroup$ – Tobias Wärre Mar 12 '15 at 7:27
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    $\begingroup$ How big is this doohicky? Size of a warehouse, size of a mountain, size of a continent? $\endgroup$ – Steve Jessop Mar 12 '15 at 8:57
  • $\begingroup$ Related: The Star Trek Voyager episode Natural Law has something like that. An entire section of the planet is shielded by an impenetrable force field, the inhabitants remained in an tribal stage while the outside world was industrialized. $\endgroup$ – Lars Ebert Mar 12 '15 at 10:11
  • $\begingroup$ @LarsEbert I actually have seen that episode but haven't thought of that... But that was an entire continent, which is what I want to avoid. $\endgroup$ – k_g Mar 13 '15 at 1:55
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For the conditions you've given (medieval technology, little to no wood, and no animals) people will really need to stay put. There isn't going to be much exploring. Without game to hunt, travellers would need to rely on carried food or foraging. Travellers will also be walking. Using this information it's not difficult to guess places already inaccessible to these humans.

You could start by looking at places humans in our own history were not able to get to with medieval technology. Places that were not explored until recently in our history or even now remain mostly unexplored. For instance:

The southern pole of inaccessibility. Antarctica wasn't even landed on until the early 19th century and the pole wasn't reached for another 100 years. Building a station at the pole, especially one disguised as a pile of snow, is unlikely to be discovered. Without wood for ships or dogs to pull sleds, even if people were able to land on Antarctica, they're not going to walk to the pole.

Under the ocean. Perhaps at the oceanic pole of inaccessibility, or at the center of the Atlantic ocean, though that seems like the likely place for fish to congregate. Under water may not initially seem like the best place for an atmospheric refinery, but only the bulk needs to be under water. With floating air transfer stations the entire facility could be disguised as nothing more than a patch of seaweed. Assuming anyone ever even comes directly across it, which is highly unlikely, they likely won't be able to investigate much. Especially considering the minimal sailing that would be going on with so little wood for ships or energy dense animal meats for distant voyages.

Mt. Everest. Just lop the peak off and replace it with the station of a similar shape. People of medieval technology won't likely reach the summit. The only reason this might backfire is because the station is belching oxygen, removing the requirement for tanks.

For all of these options it's most likely that if they were found, it could be by a small group of intrepid explorers or a lost ship at sea. In that case build in some minor automated defenses to the station. Groups of people going missing while doing crazy things will not raise any eyebrows.

In fact, put it in the Bermuda Triangle. It could be that we were the ones thrown back to medieval age technology, the rest destroyed, and the Bermuda legend arose because the off-worlders hadn't yet removed their ancient air processing equipment that brought our Earth back from the brink of being uninhabitable.

Alternatively spread it over the entire globe in the form of nanobots. This will increase the surface area for processing and will remain hidden from medieval tech due to their nanoscale size.

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    $\begingroup$ Well, I don't like the South Pole Idea as much as the others, (I was thinking that because of wild climate changes, it might be habitable) but the ocean idea sounds interesting. $\endgroup$ – k_g Mar 11 '15 at 23:23
  • $\begingroup$ @k_g if Antarctica is warm enough for low-tech life, put it in a desert. $\endgroup$ – Chris H Mar 12 '15 at 17:50
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Their solution is to put a refinery on the planet where they use large amounts of energy (they have large amounts of energy) to collect CO2, CH4, and other atmospheric pollutants and re-release harmless O2 and H2O.

This isn't really feasible. Atmospheres are mind-boggling huge.

Earth's atmosphere weighs roughly 5.5 quadrillion tons. Say you can process 1 billion tons of air an hour with your refinery, which I think is wildly optimistic. That refinery will take 627,854 years to fix everything if you never process the same air twice. Realistically it will take even longer.

A better solution that also helps with your "how do I hide this" problem is to think organic. Instead of creating mechanical factories, why don't you make plants that do that processing for you? Think about genetically engineered algae, grass, or trees that clean pollutants as part of their natural lifecycle. Because these will grow and spread across the entire planet they will end up processing far more than any mechanical solution. And you don't need to worry about hiding them - you can even make them desirable, maybe they also provide food. Then your low-tech people will help spread the plants for you.

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  • $\begingroup$ Interesting answer. I was thinking a huge refinery that would be able to process a huge amount of atmosphere, but I guess organic is probably a better idea. I already have an idea for "artificial lighting" accomplished by some sort of bioluminescent bacteria left behind by the technologically advanced people... $\endgroup$ – k_g Mar 12 '15 at 1:02
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    $\begingroup$ You're assuming that the whole atmosphere has to be processed, but that's not necessarily the case: Maybe it's enough to remove more CH4 that is produced elsewhere. Or maybe it's enough to "clean up" near some hotspots around the globe where a lot of CO2 is emitted. (population centers? active volcanoes?) That would make hiding the plants harder though, because they had to be near the hotspots. $\endgroup$ – b_russel Mar 12 '15 at 11:39
  • $\begingroup$ Are you thinking big? Air: 1 kg per cubic meter. Speed of sound: 340 m/s (rough limit on speed of a vacuum/turbine). So 1 m^2 tube can process ~340 kg of air/second, or ~1 million kg/hour, or 1000 tonnes/hour per m^2 opening (1e3) (which would be a really loud opening). Manhattan is just under 100 km^2, or 100 billion m^2 (1e11), so if your intake was that size and operated at speed of sound velocities (!), you'd process 1e14 tonnes of air/hour. The atmosphere is 5.5e18, so you process 0.02% of the atmosphere's air per hour, or 0.4%/day. In short, think big. $\endgroup$ – Yakk Mar 12 '15 at 19:37
  • $\begingroup$ @Yakk: That requires sucking in and processing all the air (from the surface out to space) of an area 200 miles square each hour. It's likely that moving that much air, at the speed of sound, would impact world-wide weather patterns, create mega storms, and kill any remaining birds and likely many insects. Also, while you can pull it in that fast, I'm not convinced you can process it that quickly and remove the pollutants. $\endgroup$ – Dan Smolinske Mar 12 '15 at 19:48
  • $\begingroup$ @DanSmolinske Sure, but that is a weather problem. The point was that a civilization slightly more advanced than ours could build an air processing plant that handled the kind of volume required to process the atmosphere in time periods approaching decades or centuries or even mere millennia. I just sketched an upper limit -- given that it was processing the entire planet's atmosphere in a year (assuming no replacement), we don't need to match those specs. Within a factor of 1000 or so would be enough. And yes, it would be a challenging project. $\endgroup$ – Yakk Mar 12 '15 at 20:11
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I would simply put them (you would want more than one, possibly many more) in the middle of an ocean. The Pacific and the Atlantic have plenty of space a medieval society would not really miss. The air processors would reasonably cause local weather similar to a permanent thunder storm with simply their waste heat. The storms would make ships naturally avoid the areas, hide the processor from distance, make it unlikely anyone seeing the processor would ever return home, and make any expedition to investigate the strange castle with 100 meter walls rumored by superstitious sailors almost certain failure.

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  • $\begingroup$ Yes - an Artificial Island with roaring thunderstorms and currents around. Any ship coming remotly close will be crashed, if it even gets there (little wood, no engines) - And the Factory inside this storm is also not really visible to anyone, hidden by clouds and just too far away from any coast $\endgroup$ – Falco Mar 12 '15 at 10:35
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What if the processing plants were in blimps or a more advanced versions thereof. The water they create could be exuded as a vapor, creating a cloud cover to hide them from view. They could travel to any hot spots that need immediate treatment, and could also be sent to areas suffering drought to provide additional water.

The blimps could even become part of the people's mythology; the Cloud Gods that appear when the air gets thick or the ground gets dry.

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You might start by creating a superstition, and back it up by a setup consisting of motion detectors and infrasound.

The motion detectors would help in activating the infrasound only when people are approaching, thus allowing the local fauna to exist undisturbed. You could let it work on animals, too, if you like, since seeing that even animals avoid the region might even strengthen your superstition. On the downside, it could be a cause of curiosity.

According to Wikipedia, Infrasound will inflict all kinds of bad feelings, and might even cause ghost sightings. This should make sure anyone doubting the superstition and trying to see for themselves should be scared off.

If necessary, a second line of defense might be an actual weapons array, making sure anyone making it through the infrasound barrier does not return to tell the story.

Combined with the scarcity of ressources and the resulting scarcity of explorers (they would be busy surviving most of the time) this should provide the secrecy you are looking for.

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I disagree with the Everest answer--people have climbed Everest without oxygen. You can't spend long in the death zone but some people can go there.

Likewise, while Antarctica wasn't explored at that tech level I don't believe it's inherently inaccessible to that tech level. Nobody went there because they had no reason to go to the extreme effort, not because it was impossible.

Instead, I suggest an environment too hazardous to be approached at the tech level. Take a volcanic island (Hawaii, perhaps??), beef up the volcano and redirect the lava. Your atmosphere plant is on the mountain, there's a moat of lava around the mountain. (The moat is lined with something that won't melt at lava heat and is enough of an insulator that the rock on the other side won't reach the melting point.) Lava keeps pouring from the volcano into the moat, it keeps spilling over the side in once place or another and then cooling.

The tech level you give can't cross a lava lake, the tremendous winds the plant will generate (in order to pull enough air through it to do the job it's going to be moving a lot of air) make balloons a suicide mission. Even if they knew there was something interesting they can't get there.

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  • $\begingroup$ That's very interesting. However, wouldn't the lava lake cause major environmental problems? $\endgroup$ – k_g Mar 12 '15 at 4:43
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    $\begingroup$ Everest was first climbed in 1953, oxygen was not the make or break, it was the rest of the gear. The same issue exists for Antarctica. It's an 1,800 mile trek from the ocean. It takes four months. There is simply no way that medieval tech, without animals, could make it there. Do you know what happens to lava on the surface? It turns to rock, especially when it's being cooled by tremendous winds. You know what else turns it to rock? Buckets of water from the ocean immediately adjacent. $\endgroup$ – Samuel Mar 12 '15 at 6:09
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    $\begingroup$ @k_g Volcanic eruptions are natural, and don't actually contribute that much carbon dioxide $\endgroup$ – March Ho Mar 12 '15 at 6:12
  • $\begingroup$ @Samuel: the high-tech gods have "large amounts of energy". Conceivably they could rig a system to heat their lava lake faster than it cools in the air, and faster than a sub-medieval group can pump water onto it. You're right that the problem of keeping the lava hot will require some thought from them and the author: volcanoes don't just create permanent standing lava on the surface of their own accord. $\endgroup$ – Steve Jessop Mar 12 '15 at 9:03
  • $\begingroup$ @Samuel I wasn't picturing the lava lake anywhere near sea level, just ringing the mountain. As for turning to rock--note that I specified to beef up the volcano--plenty of new lava pouring out. $\endgroup$ – Loren Pechtel Mar 12 '15 at 18:30
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The ocean is big. Like, really big. It takes up 2/3rds of the surface of the planet, and excluding coastlines and the occasional island, there is absolutely nothing useful out there, especially to low-tech people. Sure, there were groups of explorers that sailed across the ocean, but the earliest explorers stuck fairly close to the coast, and later explorers aimed for a fairly straight shop across. It may seem like an intrepid explorer may be able to see a factory out in the middle of the ocean, but actual chances of that are slim if you take into account what is actually visible; the curvature of the earth hides more than you might expect, especially if all you have is a spyglass on a ship.

Use the Pythagorean theorem to determine the maximum view distance of a sailing vessel: $h^2 = d^2 + r^2$ (or $d = \sqrt{h^2 - r^2}$, to solve for visual distance), where $h$ is the height of the vessel plus the radius of the earth, $d$ is the maximum visual distance, and $r$ is the radius of the earth. The radius of the Earth is about 3959 miles at sea level, at the equator; if a ship mast was 120 feet tall, a sailor with perfect conditions would be able to see an obvious object that is nearly level with the horizon at about 13.4 miles.

Average ocean wave height is about 17 feet high, which hides just about anything under that height. Basic camouflage (no shiny surfaces, water-colored exterior) would further hide anything. Weather created by the rushing wind would hide it as well, and would also create some weird weather patterns - patterns that a ship would be likely to avoid, or risk being capsized.

Of course, if there were only one factory, it would be a circle with a one hundred mile radius to be able to have any sort of effect on the atmosphere. As large as the ocean is, it makes sense to have not one, but dozens, or even hundreds of air purifying machines, floating all over the world. 1,000 air-purifying factories, each with a one mile diameter, could be placed around the Earth, and programmed to move with the tides but stay at least 200 miles away from any coastline. They could communicate, as well, to make sure that 'hot spots' were well covered, and that they were well placed across the planet. It would be almost impossible for any ship to ever find them. And, even if one were discovered, it would be noted as a huge storm over a strange hole in the ocean, hardly something superstitious sailors would ever want to go near.

As an added bonus for a smaller size, if a ship does get within a certain distance - say, 5 miles - the machine could simply close itself up and sink below the waves. Even if a ship came to investigate, they would never find it. As soon as the ship moves away (or it moves itself), it could rise and begin recycling again.

Finally, if one did fail, it could simply float its way to a charted deep hole and sink to the bottom, where no one would ever find it.

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You could just make it out of adamantium or some other space-age indestructible material and let the natives do whatever they want with it. They can walk right up to it, bang on it with clubs or axes, but nothing they do will even scratch the surface. After a couple generations people won't even notice it any more.
Medieval people got around. Vikings, south pacific islanders, and many other groups crossed huge amounts of ocean. Nepalese have been climbing the highest mountains since long before modern climbing techniques.
If you think about it, humanity started somewhere in the general area of Africa, and spread across the globe. There is literally nowhere that your factory will be safe from people, especially if they know it exists.

Another option is biological and wide spread. Super trees and plants, but then you have to deal with human caused deforestation and how to spread the plants, especially if a lot of the planet is arid or not plant friendly in some other way.

The last option is simply fear.
"We don't go into that place, because the monsters hunt anyone that does. No one returns!"
Fear will work more effectivly than walls a lot of the time, and for the people that are brave, you have an army of mutant spider robot attack squid to make sure they don't come back out.
The only real drawback to this is really big armies. If some king really wants your factory, and can get together a large enough army, then they might be able to overcome your giant killer super space hamsters by numbers alone.

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To take Loren's lava answer one step further, why not create or use existing human uninhabitable places to place your machines?

The volcanoes are a great example of this. You have the energy, all you need to do is put the machines inside volcanoes, perhaps even using the natural volcanic emissions as part of the process.

Further, rather than processing the air by pulling it in, fixing it, then emitting it, why not simply emit catalysts and the chemicals necessary to cause those reactions in the atmosphere?

To consider volume, note that we altered the ozone significantly over a matter of decades, but by changing what we put into the air the holes in the ozone repaired themselves in a matter of years.

So spread these machines around the world, disguised as, or inside, volcanoes that are active enough, or appear active enough, that humans will avoid them. The smoke billowing out (chemicals you create) will appear natural, and you could build in a self destruct sequence that destroys the machinery and buries it under lava if the humans become too curious. Given the recent catastrophes and being tossed back into the stone or feudal age, they probably won't think it odd for volcanoes to show up in places where there previously was little or no tectonic activity.

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