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Imagine a human observation and research facility being built on a planet in a distant solar system.

How could an alien colony exist beneath the planet's surface, staying completely invisible to humans on the ground? Let's say that aliens have a portal technology allowing them to travel from within the planet's insides to their home land at will and transporting resources back and forth - without ever touching the planet's surface.

Let's say their main goal for having the underground colony is resource extraction from the planet, and they want to stay invisible to avoid any conflicts if possible.

Could an alien colony be shielded to make it undetectable to human equipment either from afar and from up close?

Let's assume that the alien underground colony was built before humans came to the planet, so that main alien construction works were already done at that point.

Could it depend on passive sound and EMP proofing? Could it incorporate active silencing devices? The aliens still need to perform mining and ore processing inside of the planet, so that normally would be detectable by human equipment.

How could that be prevented? Using energy based mining tools instead of impact-based ones?

EDIT 1:

This question was marked as a potential duplicate of another one: How big could an alien-made object on the far side of the moon become so that discovery still can only happen by accident?

Though I think it's different, as the linked question asks about objects on the surface of the moon, while this question is about covering alien mining activity under a planet's surface.

To give more insight in my plot idea:

Humans have built an observation and research facility in a distant solar system - not knowing the planet is being mined by aliens, who have built a stealth outpost deep under the planet's surface. After a few years of operation the humans started drilling into the planet for research.

An alien colony hidden beneath have realized they cannot hide there for much longer without being discovered so they decided to attack, until they still have the upperhand of being unexpected - as evacuating and hiding all their traces was impossible to do.

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    $\begingroup$ Without much more information about the capabilities (sensors, countermeasures, etc) of the two sides I don't see how this is possible to answer. $\endgroup$
    – Tim B
    Jul 19, 2018 at 9:26
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    $\begingroup$ How many humans are on the ground? A planet is, by definition, quite large. I'm in Spain and I can't hear the noise of the bauxite mines in China, though I know they are quite noisy. Unless the humans happen to be just right above the mine and placing seismometers all over the place I very much doubt they could notice anything. $\endgroup$
    – Rekesoft
    Jul 19, 2018 at 9:27
  • $\begingroup$ @Rekesoft - I was wondering about a corner case where the alien mine is located right beneath the human colony. But an easy solution is to put it somewhere else. $\endgroup$
    – unfa
    Jul 19, 2018 at 9:45
  • $\begingroup$ Possible duplicate of How big could an alien-made object on the far side of the moon become so that discovery still can only happen by accident? $\endgroup$
    – dot_Sp0T
    Jul 19, 2018 at 11:05
  • $\begingroup$ @dot_Sp0T This is not a duplicate. That question asks about a facility on the surface of the world, the discovery of which is limited to the resolution of maps/cameras looking at it. Finding a facility underground is a different problem set. $\endgroup$
    – Frostfyre
    Jul 19, 2018 at 12:31

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The planet is crazy noisy already.

This planet has loads of seismic activity and loads of volcanism. Maybe it is like Io, with a giant Jupiter equivalent wrenching and twisting it. You can hear it up through your feet when you are standing there. Seismographs return a chaotic roar. Or maybe meteorites regularly slam into the planet. Or the mammoth rock worms - they are native to the place, and their tunneling grinding noises sound a lot like the alien miners.

With any of these noise methods (go ahead, use them all!), it would be hard for the humans to distinguish some additional mining noise from the noise already on the planet and explained to the humans satisfaction.

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    $\begingroup$ Depending much on the 'loads' part, extensive amounts of seismic and volcanic activity would render the planet difficult or even impossible to mine as well as impossible to establish observation and research facilities on the surface. The latter also counts for regular meteorite impact, and depending on the mine's depth as well as the size of the meteors might regularly cause the mines to collapse. Rock worms might be an option but even those might stumble upon the mines by accident constantly roaming the area. $\endgroup$
    – Otto Abnormalverbraucher
    Jul 19, 2018 at 14:42
  • $\begingroup$ Also there are means of detection other than simple noise. $\endgroup$
    – Otto Abnormalverbraucher
    Jul 19, 2018 at 14:42
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    $\begingroup$ @OttoAbnormalverbraucher: alien miners are not wusses. They get buried alive all the time and wait to get dug out. Surface researcher humans are not wusses either but they stay nervous. $\endgroup$
    – Willk
    Jul 19, 2018 at 16:45
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    $\begingroup$ Or, heck, just have the mining operation be in place long before the humans get there. This planet has some really weird seismic activity, but it's been there as long as humans have had sensors examining it, so it must be normal, right? $\endgroup$
    – anon
    Jul 19, 2018 at 17:13
  • $\begingroup$ I thought I'd try to avoid meteorite strikes, or I'd have to add some active shielding to the human colony - however that could be an interesting factor! $\endgroup$
    – unfa
    Jul 20, 2018 at 7:34
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Considering the signals that an underground mining activity will produce, we can speculate on how to shield them (and what tech level is needed on the human side to detect them).

Gravitational effects

Removing large quantities of minerals from underground would reduce the local density of the crust. This would have a measurable gravitational effects, provided that there are atomic clocks and satellites.

Since gravity cannot be shielded, the only way to hide this is to avoid leaving hollow places and filling back the mines with something denser than air (the closest to the density of the abducted material, the better). P.S. This can explain why alien worlds are always so tidy, they have a garbage dump off their backyard.

Noise

Mining, if one uses what we human use, is going to be noisy. And a "clang clang tump" noise coming from underground would be noticed even by very primitive humans.

Active noise suppression might be a thing, or they could simply swipe the portal around, transporting large carrots to their home planet for further processing. Using the a second portal to send back the filler, as mentioned in the above point, would limit ground collapse, improving stealth effect.

Underground circulation effects

Depending on when the mining takes place, it can impact some aquifer, disrupting its circulation and maybe impacting also wells and springs above the surface, or, even worse, cutting through some touristic place like caves.

I guess for this careful prospection before mining is the only viable solution.

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An underground mine would remain undetected pretty much without any special precautions. Assuming they're not using heavy explosives, or anything like that, just drilling and excavating.

The ground is extremely good at blocking EM, so it will be safe from any scanners of that ilk. Same goes for acoustics, the noise will not couple well with air, so it'll be pretty much silent to any observer not specifically looking for it (you'd need to have sensors in/on the ground).

Biggest worry would be sensitive seismometers. For this reason you would want to avoid explosives, so you don't cause large scale shockwaves in the ground. Best defense will be distance, so you would want to locate your mine at least some tens of kilometers away from any settlement or geologically "interesting" location to be sure. The closer it is to any seismometers, the higher the risk become that it shows up and somebody gets curious.

Once someone gets curious the way you get found is via active echo mapping. Ground thumpers or explosives to generate shockwaves, the echos of which are picked up by sensors. The mine would show up as unexplained voids.

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Planets are big. Incredibly big.

Sure, the colonists would have advanced sats in orbit, and advanced expert systems to analyze the imagery for clues, but there are likely many false positives to check out. Consider the Cydonia Face, the Baltic Sea Anomaly, the Nazca Lines, and the Bimini Road. Only one out of four of my semi-random choices is generally considered to be artificial ...

Depening on the technology and orbital infrastructure, you might not even have to assume teleportation technology. With present-day technologies the superpowers are somewhat capable of detecting missile launches. Even so, the coverage of Russian missile fields and presumed SSBN bastion areas would be better than the coverage of Antarctica. It would be conceivable that a colony might miss a shuttle launch on the other side of the planet, especially if the aliens have "magical" drives without exhaust plumes.

You would have to assume that the aliens tried to avoid visible surface structures, but imagine e.g. a mine entrance under the trees of a jungle and "seaplane" shuttles landing on rivers, or a door in a cliff face.

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    $\begingroup$ I was just going to write up something along these lines myself. You might want to add that people usually don't find things if they are not looking for them. If there is evidence of mining like unusual seismic waves, noone will find it unless someone is specifically trying. There is a LOT of science to do, and no chance of the science station doing all of it. $\endgroup$
    – BobTheAverage
    Jul 19, 2018 at 18:15
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If the humans are moderately careful, the aliens will be found.

The aliens have to establish their underground mining colony in the first place. Unless they have such a high level of handwavium technology that they can blindly teleport to a location underground and excavate it in the process, they must have started on the surface. This means that they tunnelled down from somewhere near where the humans now have their base.

The use of ground penetrating radar has become standard in both archaeology and military surveying. While GPR has a limited penetration depth, it can easily identify disturbed earth near the surface - this is how it is used in archaeology to identify previously dug areas compared to untouched ground. (It is also used in murder investigations to identify disturbed ground that may be a grave.) Before setting up their research base, the humans would conduct a GPR survey of the surrounding area, both to identify any signs of previous occupation and possible current threats (like large tunnelling alien lifeforms). This will also probably lead to identifying tailings piles.

The other means by which the humans are likely to locate the alien mining colony, especially once they detect signs of excavation activity, is to set up an array of seismometers and set off some mapping charges. By measuring how the ground waves propagate through the area they will be able to locate any large caverns, or even areas with significantly different conduction of ground waves.

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    $\begingroup$ The question is, why would they use GPR in the first place? Sure, we use it for archaeological and military purposes but other than that? If there is no hint on landmines or possible archaeological findings, there is little reason to use one. $\endgroup$
    – Otto Abnormalverbraucher
    Jul 19, 2018 at 14:48
  • $\begingroup$ @OttoAbnormalverbraucher these humans are setting up a base on a previously unsettled planet and have probably seen at least one movie in the Aliens series. They will take every possible precaution to make sure they are not setting up camp on top of the equivalent of a bull ant nest. However, surveying the rest of the planet will be lower priority - if the aliens have their mine somewhere else on the planet it probably will not be detected. $\endgroup$
    – KerrAvon2055
    Jul 20, 2018 at 9:19
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    $\begingroup$ My guess is they'd check whether the planet is uninhabited from outside space by surveying and probing but if they don't find anything on the surface I highly doubt they'll start scanning kilometers deep into the crust to make sure not to absolutely miss anything. Even if they found a hint of intelligent alien life on the surface they'd be more likely to dismiss it as remnant of the past rather than check, whether the aliens have gone underground. Decades later, after the basic provisions have been set up, trying to find more about the alien's history that might have been buried - maybe. $\endgroup$
    – Otto Abnormalverbraucher
    Jul 20, 2018 at 13:24

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