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I need to know if it’d be possible for a underground ring-like structure of immense proportions and alien design to go unnoticed by modern day sonar, radar, and geological surveying.

For an idea of size, imagine a tunnel about the width of a football field or bigger (maybe 100-200 meters in diameter) that’s buried 2- 5 kilometers underground and wraps around the entire planet (giving it a circumference of 40000 km).

Could something like this remain hidden until a mining project accidentally uncovers it in Africa, or would something this size have been detected by now with the technology developed in the 20th and 21st centuries?

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    $\begingroup$ When would it have been built? Even within a single lifetime, tectonic plates move up to a meter. $\endgroup$
    – geometrian
    Commented Oct 24, 2017 at 4:33
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    $\begingroup$ I believe there's one below a city in US $\endgroup$
    – Vylix
    Commented Oct 24, 2017 at 7:24
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    $\begingroup$ Where did they put all the excavated dirt and rock? That is a LOT of stuff to move around. Maybe that is where the moon actually came from? Tailings from this tunnel? $\endgroup$
    – Justin Thyme
    Commented Oct 24, 2017 at 15:23
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    $\begingroup$ @JustinThyme - where did they put the dirt and rock? Why, outside the tunnel, of course :-). 400ft x 400ft x 40000km = 3.34E+11 cubic meters. The rockies are 4800 km long and range up to 300 miles wide. Say average it at 200 km wide, you have an area 9.6E+11 squared meters. Sprinkle the dirt from the tunnel over the rockies and you raise their height by 30 cm or so. Heck, sprinkle it all over earth's landmass, to avoid messing with earth's center of gravity too much, and you'd raise the ground level by less than a millimeter. $\endgroup$
    – user11864
    Commented Oct 24, 2017 at 16:55
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    $\begingroup$ @JustinThyme and on a planetary scale, 6 mount everests are not much. Consider that if you draw a circle, as big as possible on your computer screen (2k pixels maybe), Mount Everest, from base to peak, would be a blip about half a pixel tall. The point is, especially compared with the technological hurdles for drilling that deep and maintaining such a structure, disposal of the excess dirt is the least of your problems. $\endgroup$
    – user11864
    Commented Oct 24, 2017 at 20:53

10 Answers 10

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Consider the following:

  • The diameter of the Earth is 12,742 kilometers.

  • Our planet's crust thickness is anywhere from 30 kilometers on the continents, to 5 - 10 kilometers on the ocean floors (which have not been explored or mined in any real detail).

  • The deepest mine on the planet goes down less than 4 kilometers.

  • Your average ground penetrating radar does so at a depth of 30 meters (this will obviously vary with the system and material it has to travel through).

Keeping all this in mind, I wouldn't be surprised if such a tunnel did in fact exist. Even with today's technology, it might not be found for hundreds of years, if ever.

Remember, that even if your detection technology is pretty amazing, you still have to be looking at relatively specific location to find it. We don't go around ground scanning every random location on the planet, especially not at any considerable depth.

Simply place it slightly deeper than we've gone, and you're golden. In the not too far future advances in material science and automation may result in robots being built which can mine to greater depths than we've been able to achieve so far, and stumble onto it.

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    $\begingroup$ +1 for putting the problem in perspective. Just a note: the tunnel should not come anywhere near an oil field, or an area suspected to have oil, or any other area where systematic seismic surveying is done. $\endgroup$
    – AlexP
    Commented Oct 24, 2017 at 2:32
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    $\begingroup$ It's true that ground radar penetration is short, but seismic methods are a lot more penetrating, specially when analyzing seismic waves from natural earthquakes. We need just to realize that seismic techniques have been used to map the Earth's core. $\endgroup$
    – Pere
    Commented Oct 24, 2017 at 8:49
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    $\begingroup$ @Pere: not to 100-200 m resolution though. That would be like finding a hair in your soup by tapping the bowl with a spoon. $\endgroup$
    – Joe Bloggs
    Commented Oct 24, 2017 at 9:08
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    $\begingroup$ I have to echo the criticism, this answer is just wrong. Ground-penetrating radar is a straw example, that's only used for investigating details of small structures near the surface (except in some special cases; e.g. in a ice shelf, GPR reaches much further). Geophysicists have plenty of other methods, most of which reach much further, and though they indeed don't offer such a great resolution as GPR, it would be plenty enough for noticing a 100 m cavity. $\endgroup$
    – leftaroundabout
    Commented Oct 24, 2017 at 12:35
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    $\begingroup$ @JoeBloggs “Hair in a bowl of soup” is not comparable at all, because a) that's vastly exaggerating the size proportions (Øhair:bowl ≈ 1:1000, vs. Øtunnel:depth ≈ 1:50), b) the hair has roughly the same density as the soup around it and can therefore just transmit any waves, whereas the tunnel has a completely different density from the surrounding matter, which means it would strongly scatter any seismic waves. Indeed you would not be able to exactly measure the tunnel from the surface, but you'd damn sure notice there's something extremely funny down there. $\endgroup$
    – leftaroundabout
    Commented Oct 24, 2017 at 12:35
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The Earth is better surveyed than is generally realized. It is probable that geophysical survey techniques would have revealed the existence of an underground megastructure.

Geophysical surveys may use a great variety of sensing instruments, and data may be collected from above or below the Earth's surface or from aerial, orbital, or marine platforms. Geophysical surveys have many applications in geology, archaeology, mineral and energy exploration, oceanography, and engineering. Geophysical surveys are used in industry as well as for academic research.

There is a wide variety and considerable spectrum of instruments used for geophysical surveys.

Seismic methods, such as reflection seismology, seismic refraction, and seismic tomography.

Seismoelectrical method

Geodesy and gravity techniques, including gravimetry and gravity gradiometry.

Magnetic techniques, including aeromagnetic surveys and magnetometers.

Electrical techniques, including electrical resistivity tomography, induced polarization, spontaneous potential and marine control source electromagnetic (mCSEM) or EM seabed logging.

Electromagnetic methods, such as magnetotellurics, ground penetrating radar and transient/time-domain electromagnetics, magnetic resonance sounding (MRS).

Borehole geophysics, also called well logging.

Remote sensing techniques, including hyperspectral.

Source: Geophysical survey

Many of these instruments and techniques could reveal a deep subterranean structure depending on the nature and construction of the megastructure.

For example, because of its depth the world-girdling tunnels would need to be made of extremely strong and, possibly, dense materials. Gravimetric surveys might detect its presence. Magnetomtric surveys would the same if its construction material(s) were highly magnetic.

Remote sensing imagery could show a continuous band of disturbance or its remnants if earth rock and soil had to be displaced for the megastructure's construction and installation. Building something on this scale at a depth of kilometres will leave its mark on the planet.

Remote sensing, orbital and aerial imaging are excellent at uncovering changes to the surface and subsurface of planetary terrain. Changes or structures that wouldn't be apparent if you were standing in those places yourself.

The ring megastructure is likely to be detected by variations in mass distribution using geodetic remote sensing.

Geodetic remote sensing can be gravimetric or geometric. Overhead gravity data collection was first used in aerial submarine detection. This data revealed minute perturbations in the Earth’s gravitational field that may be used to determine changes in the mass distribution of the Earth, which in turn may be used for geophysical studies, as in GRACE (satellite). Geometric remote sensing includes position and deformation imaging using InSAR, LIDAR, etc

There is also seismic mapping of the Earth's interior which might determine there was two hundred metre diameter tunnel around the world.

While a mine or a bore hole breaking through or hitting the megastructure is more hit or miss geophysics has an arsenal of methods and instruments to survey what is down below the surface of our planet.

In summary, there a wide range of geophysical survey methods. Many of which, both singly or together, could indicate the presence of a subterranean megastructure. Once its existence was suspected the range of geophysical instruments could be brought to beat to further elucidate it. In principle, puny humans have the techniques to find a deep megastructure and have had them since the twentieth century, therefore, they can only be expected to have gotten better this century. This makes the discovery of the megastructure highly probable.

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I must disagree with AndreiROM on this. I believe it would have been found some time ago.

I'm thinking specifically of oil exploration. They go all over the place looking at subterranean layout to figure out where it's worthwhile to drill for oil. If it's drillable they're going to have a look--and your loop is certainly going to pass through some drillable areas.

About the only way I think it could be missed is if it never went near geologic features that could possibly contain oil. It still would have been seen but it might not be noticed if the survey crew stopped looking because they knew it wasn't oil. (The Chicxulub crater was missed this way--the oil guys saw it in the 1950s but dismissed it as volcanic, no chance of oil, they never looked at it closely enough to realize what they were seeing.)

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    $\begingroup$ There's a point to be made here. In 2009 the Deepwater Horizon drilled the deepest well at 10Km. However, the OP need only push his creation deeper still to solve the problem. Further, Adrei's point about needing to look at a specific location could still keep it hidden. Given the OP's design, it's a needle in a haystack. Great point, though. $\endgroup$
    – JBH
    Commented Oct 24, 2017 at 2:58
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    $\begingroup$ If I were to plan such a megastructure, I would try to avoid anything near oil to not have to fear a problem of that stuff seeping in. $\endgroup$
    – PlasmaHH
    Commented Oct 24, 2017 at 11:04
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    $\begingroup$ "Hey, there's a weird reading; a straight line running due east-west." "Oh? Right, that. Yeah you see that a lot at this latitude; it's just air. But that patch over there might be worth something." Airplane pilots were seeing colored lightning off the tops of clouds for 50 years before telling anyone who cared. $\endgroup$
    – user25818
    Commented Oct 24, 2017 at 16:30
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    $\begingroup$ @PlasmaHH It's not just oil, but anything the oil guys would think might contain oil. $\endgroup$
    – Loren Pechtel
    Commented Oct 24, 2017 at 17:11
  • $\begingroup$ @JBH Deepwater Horizon was the deepest oil well; other exploration borings have gone as deep, the deepest being 12.2 Km (the Kola Superdeep Borehole). I was thinking that this might be deeper than was useful, given it's nearly halfway through the Earth's crust, but perhaps the heat from the mantle is being used as a energy source. $\endgroup$
    – brichins
    Commented Oct 25, 2017 at 1:44
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You have a very big problem with your 2-5 km. depth. The average depth of the Atlantic ocean is over 3 km. I suspect it would have to be deeper than 5 km., and it would not be a perfect ring.

And forget about around the equator. If you went pole to pole, along roughly 30 degrees west - 150 degrees east longitude, you pretty much miss all land mass and drilling sites. It's ocean all the way, except for a piece of Australia. Since we have done very little in the way of exploring this region, it would be virtually undetectable by current methods. It misses a lot of volcanic activity as well.

Tectonic plate shifting would be a problem no matter where you put it. There would be tremendous sheer forces between plates.

Earthquakes can be as deep as 700 km. or more.

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    $\begingroup$ This is a comment, not an answer to the question. $\endgroup$
    – MichaelK
    Commented Oct 24, 2017 at 7:49
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    $\begingroup$ I was about to answer the same, so I will upvote and comment here. @JustinTyme: you may want to edit your answer to be a bit more "answer-like", something along the lines: "It may remain unnoticed if... but structural integrity would be difficult due to tectonic plaques movement...". It is interesting to note there's a Earth circumference (cape Horn, Africa Pakistan, Corea) where plate movement is roughly along the circle ;) $\endgroup$
    – ZioByte
    Commented Oct 24, 2017 at 9:06
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    $\begingroup$ @ MichaelK Using the 30 degree west - 150 degree east longitude is an answer and a solution. A virtually undetectable place to put such a ring. $\endgroup$
    – Justin Thyme
    Commented Oct 24, 2017 at 15:15
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    $\begingroup$ If you don't mind it being torn apart by crossing multiple spreading centers, subduction zones, and plates moving in opposite directions. $\endgroup$
    – Keith Morrison
    Commented Oct 24, 2017 at 22:50
  • $\begingroup$ @ Keith Morrison Maybe they are residents from Myst. $\endgroup$
    – Justin Thyme
    Commented Oct 25, 2017 at 1:19
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We have a pretty good idea of the general layout of the Earth core from seismic data. Seismometers are used to track earthquakes and nuclear tests. These models have been developed by reconciling theoretical physics and experimental data.

So if there was such a tunnel, and if the tunnel is under the equator, it might show up in the model as the "equatorial anomaly" or "equatorial subduction zone." Yes, there are strange echoes. No, they can't possibly be a tunnel. A density anomaly. Some physicist comes up with an explanation, the explanation fits the experimental data, the theory is provisionally accepted, and mankind has wrong ideas about the contents of Earth.

Say they think the core is (or was?) more fluid than we think, and coriolis forces must have concentrated high density (or low density) materials in patterns, much like the cloud bands of Jupiter.

Alternatively, the data gets explained away as something like a mascon.

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    $\begingroup$ Yes, however this would be a very significant anomaly, and unlike a mascon we could use plenty of different methods to investigate it. Perform a joint inversion of gravimetry, seismic and magnetotellurics, and you'd see that it's very sharply localised and with very exotic properties. That would easily warrant a large-scale drilling operation to nail down what's up there†, even if it would be very expensive. Pretty sure some country would have done that no later than 1980 (Kola reached 12 km in 1979). — — — — — — — — — — — — — † (Or should I say: “to screw up what's down there”?) $\endgroup$
    – leftaroundabout
    Commented Oct 24, 2017 at 12:14
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    $\begingroup$ @leftaroundabout, there is some merit to what you say, but in order to define an anomaly you need a baseline for normal. If the megastructure predates scientific investigation, it is part of that baseline. Consider just how sharply the ridges of the moon Europa are drawn -- we are quite confident they are natural. $\endgroup$
    – o.m.
    Commented Oct 24, 2017 at 15:27
  • $\begingroup$ That's....that's not how it works. A negative density anomaly, which such a tunnel would be, arranged in a linear fashion circling the entire globe would immediately raise issues because common sense would indicate it shouldn't be there. Look at it this way: if you saw a tunnel composed of nothing but air going for kilometers through a lake, would you assume that's "natural" just because it had "always been there"? Probably not, because all your experience with water everywhere else tells you it shouldn't be there. This tunnel would cause the same issue. $\endgroup$
    – Keith Morrison
    Commented Oct 24, 2017 at 22:55
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    $\begingroup$ @KeithMorrison, the data wouldn't show an empty tunnel. It would show strange reflections that would be consistent with a tunnel, or some other weird rock formation. $\endgroup$
    – o.m.
    Commented Oct 25, 2017 at 5:10
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    $\begingroup$ @o.m. That's not how modern seismic surveys work. Analysis of the pressure waves that are generated by the source and picked up by the geophone array take into account not just reflection of the wave but the way the wave refracts when passing through materials of different density as the velocity changes. Passing through a long, linear void in otherwise solid rock would show a "layer" of such low density that it should be impossible, and therefore would immediately attract attention. A detailed survey would be able to determine that is was, essentially, empty space. $\endgroup$
    – Keith Morrison
    Commented Oct 25, 2017 at 16:36
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If I were the aliens and wanted to hide the tunnel I would align this tunnel with the mid-ocean ridges.

https://en.wikipedia.org/wiki/Mid-ocean_ridge

The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every ocean, making it the longest mountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long.[6]

mid ocean ridges from wikipedia

The ridges are formed by the motion of plates away from each other. It is not a symmetric ring but it is ringish; the real ridges do go around the earth, mostly, in a somewhat meandering way. Especially that Indian ridge. A structure on one side of the plate boundary would be carried away from the boundary, but the accumulated stone there should not flex or distort too much. Clearly though, unless the aliens built this thing just 6,000 years ago, the engineering of the tunnel is going to have to accommodate some flex. I think a civilization capable of building such a thing would probably not make it out of mud bricks. Some sort of crystalline structure capable of compensating for gradual movement is not outrageous.

The mid ocean ridges are deep and not well explored.

http://oceanexplorer.noaa.gov/facts/mid-ocean-ridge.html

Like the rest of the deep-ocean floor, we have explored less of the mountains of the Mid-Ocean Ridge system than the surface of Venus, Mars, or the dark side of the Moon. Use of submersible or remotely operated vehicles to explore the mid-ocean ridge has provided information on less than 0.1 percent of the ridge!

If someone were examining at a ridge, a (barely seen) deep geological feature aligned with the long axis of the ridge would likely be assumed to be natural in origin.

I like the idea that the persons discovering this tunnel system realizes that its makers not only bored out the tunnels but also engineered the plates themselves to serve their purposes. Tunnel explorers: watch out for shoggoths!

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    $\begingroup$ Tekeli-li! Tekeli-li! $\endgroup$
    – Kieran
    Commented Oct 24, 2017 at 16:12
  • $\begingroup$ @Will "but also engineered the plates themselves to serve their purposes." that would require the makers to have engineered the plates billions of years ago. $\endgroup$
    – RonJohn
    Commented Oct 24, 2017 at 19:34
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    $\begingroup$ You overlooked the obvious cause of the ridges - they're a side effect of the tunneling technology! The power to the maintenance field surrounding the tunnels was slightly mis-calculated by a summer intern, resulting in a miniscule gravity overload which is slowly but constantly sucking the plates together into a ridge over the tunnels, and tearing them away from the divergent plate boundaries. ...or maybe it was deliberate, and the intern wanted a self-destruct mechanism slow enough to not implicate him until he had a new job. $\endgroup$
    – brichins
    Commented Oct 25, 2017 at 1:52
  • $\begingroup$ ... and of course, you also forgot the secondary set of access / maintenance tunnels running underneath the continental divides. The Himalayas and the Chilean Andes obviously have either redundancy or larger capacity, and Antarctica and Greenland are revealed to be major connection hubs, no doubt with additional climate-control measures taken to make those areas undesirable for surface habitation (and the pesky exploration drilling that comes with it). $\endgroup$
    – brichins
    Commented Oct 25, 2017 at 2:03
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    $\begingroup$ @RonJohn - having engineered them does not mean they made them. People engineer mountains to make it easier for railways to traverse them. Those engineers got to the mountains some time after they were formed. Likewise the plates and these alien engineers. $\endgroup$
    – Willk
    Commented Oct 25, 2017 at 12:07
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It would have been found decades ago.

Earthquake surveys act as a deep level sonar and show up large structures inside the earth. This allows for discovery of the inner and outer core, good estimates about their structure and chemical composition. The mantle has a velocity change called the Morohovicic discontinuity that is due to small changes in density that was found over a century ago https://en.wikipedia.org/wiki/Mohorovi%C4%8Di%C4%87_discontinuity

A tunnel full of air would have a very sharp density change and this would give rise to sesmic wave reflections and refractions and these would show up on large scale surveys and data collections. The same technique is used these days to look ad descending plate fragments and these only have subtle density changes - an air filled tunnel would stick out like a beacon. Whilst not something that could be easily investigated at the time - this would have been found in the early part of the 20th century.

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    $\begingroup$ +1: Even if it wasn't full of air, the fact that it's a single continuous structure would have seismic vibrations traveling along it at a constant speed that's very different to the rock around it. This would be noticed, unless it was specifically built to dampen seismic vibrations, essentially it would ring like a bell. As $\endgroup$
    – Binary Worrier
    Commented Oct 25, 2017 at 11:28
  • $\begingroup$ Also, if it deforms / breaks with seismic activity it won't remain a ring for very long. Also it will "break" differently to the rock around it, making it obvious that there's something there. If it doesn't break then it's going to be even more conspicuous, as the forces will move around it, showing that there's something very odd happening in that location. $\endgroup$
    – Binary Worrier
    Commented Oct 25, 2017 at 11:31
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Flood it

Make sure the tunnel runs almost exclusively underwater and flood it with seawater. Seismographic equipment and measurements probably won't detect it, likewise any "deep" radar technologies that might exist in your world. It'll basically blend in.

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  • $\begingroup$ It will be detected. A seismic study will trivially pick up the difference in density between a granite pluton (average specific gravity 2.75) and the basalt it's in (average specific gravity 2.9) without trying hard. Picking out the difference between rock and a tunnel full of water (specific gravity about 1) is child's play. $\endgroup$
    – Keith Morrison
    Commented Oct 27, 2017 at 20:55
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If it is on earth, or being observed by mankind; generally speaking people ignore what they can't easily explain or try explaining with old ideas rather the. Form new ones. In a sense you could have this mega structure above ground and covered with half a mile of rock and have it sit long enough and everyone would consider it normal or explain it away. Vise versa those who who look for contact generally see it everywhere.

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They could have the TARDIS effect and be in a tiny sphere.One would think that they could even be under a desert with some advanced shielding.

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  • $\begingroup$ Welcome to Worldbuilding! As this question is a reality-check question, the preferred answers would generally be 'harder-science' (much more realistic) than the likes of the TARDIS, unless specified otherwise in the question, so would you be able to edit your answer to take this into account? Thanks $\endgroup$
    – Mithrandir24601
    Commented Oct 25, 2017 at 23:15

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