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Suppose you need to dig underground through solid rock to build your secret base. You have a drill with stealthy and efficient Future Tech but other tech is mostly present day.

What do you do with the material you removed? Where does it go? My thinking is that you could compress the quartz into Stishivite is metastable at room pressure with a density of 4.3 g/cc (about 65% denser than quartz at 2.65) and is the second densest form of silica (not sure how stable the densest form is, it is very rare).

But you would still have to excavate and compress about three times (assuming mostly quartz based rock) the volume you end up living in. How best to manage all this material while keeping it underground? A growing centralized dumping chamber or maybe just using it to line the walls of the tunnel?

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    $\begingroup$ if you have the technology to silently make stishivite you can just push through the solid rock like an earthworm through loose soil. The pressures needed to make it are normally only found in meteorite impacts. $\endgroup$
    – John
    Commented 2 days ago
  • $\begingroup$ @John that pushing would definitely be detectable. Not that regular drilling wouldn't be but I'd expect exerting this much force would increase detection range drastically. To the point where "detection" likely means" anyone standing on the ground in a 50km radius $\endgroup$
    – QuestionablePresence
    Commented yesterday
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    $\begingroup$ How deep are we drilling? How much are we drilling? Through what are we drilling? $\endgroup$
    – JBH
    Commented yesterday
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    $\begingroup$ @Anketam: Yes and current tech can do both. But there isn't a big market for Stishivite gemstones given it looks just like quartz. $\endgroup$
    – Kevin Kostlan
    Commented yesterday
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    $\begingroup$ your best bet is to do what the US military did when they needed ot build an underground base in secret, build something else over the entrance and hide the digging as part of it. AKA the Greenbrier approach. this lets you hide everything, power, machinery, personel movement, and of course excavated material. $\endgroup$
    – John
    Commented yesterday

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Remember, stealth is more than just being unseen. Unnoticed is really just as good. Don't make piles, instead make nice blocks and just act like you are building something. The more distracting the better. They don't have to know that the material is coming from under their own city.

Even better if you actually are building something, maybe a defensive wall against the inevitable counter-attack.

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    $\begingroup$ Won't people get suspicious when they see all these blocks appearing out of "nowhere"? $\endgroup$
    – Kevin Kostlan
    Commented yesterday
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    $\begingroup$ They won't be appearing out of nowhere, they'll be apparently arriving on trucks (with fully covered cargo trays). The trucks will mostly be empty (don't forget to adjust the suspension to make them look like they are heavily loaded on the way in and empty on the way out). And/or at least some of the trips the trucks can be bringing in the fittings for the underground base - cabling, wall panels, plumbing, structural supports etc $\endgroup$
    – KerrAvon2055
    Commented yesterday
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Build your base near an underground river

Then grind your Stishivite (or uncompressed rubble, depending on "available" discharge volume) to a powder and wash it away using the current.

Of course this requires either a massive river or slow progress as the maximum capacity before noticeable changes occur to the water are small.

Depending on the setup an aboveground river could work as well by accessing it from below in a spot with bad visibility.

[Edit addition:]

Build it starting from or attached to an old mine

Inspired by Anketam, but taking a different approach: Just use an old, existing mine either as starting point or tunnel into it in secret. Then use those usually giant caverns as a dumping ground. All you need to do is take over or bribe any company that does end-of-life maintenance or checkups on that old mine and you're good. Modern mines have already excavated insanely large caverns underground, no matter what enormous lair you want to create, it'll suffice. If you set up a Stishivite compression plant even more so.

This approach also has the bonus of allowing you to plausibly get started: Either buy the mine as it gets shut down or - after taking over the maintenance company - write up something about increasing seismic instability to keep or get your (initial) equipment down there without much scrutiny.

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    $\begingroup$ clogging an underground river is really bad $\endgroup$
    – fraxinus
    Commented yesterday
  • $\begingroup$ @fraxinus yeah you'd really need to make sure it's draining in a way that you won't clog $\endgroup$
    – QuestionablePresence
    Commented 22 hours ago
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Grind the rock into powder and mix with water. You've now got rock slurry.

You can pump the rock slurry through pipes to places people won't easily notice. Nearby lake or river? No? Then how about the city sewer? Don't feel like constructing a clandestine network of rock slurry pipes? No problem. Just set up an inconspicuous tanker truck business. Fill the tanker trucks up with rock slurry and have them dump it somewhere far away.

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This answers assumes the person making a secret base has some wealth since even hand waiving the tunneling for the secret base, would still cost a considerable amount of money to set it up and furnish it.

Frame Challenge: Buy an Old Mine or Build a New Mine

The technology proposed in the question is capable of also compressing other materials besides quartz. As such it is able to make precious gemstones like diamonds. So instead of trying to make a secret base in a random location, buy a dead diamond mine, or buy a plot of land in a more convenient location to build a new one. Then announce you have found signs of possible diamonds, and begin some exploratory digging to confirm it. Then scale it up to a fully operational mine, that instead of mining diamonds secretly makes them. An operating mine is expected to produce large amounts of spoil and so you no longer need to hide it.

The secret base would be hidden in part of the old mine or in a closed off section of the mine. While the rest of the mine looks legit enough to get past any government safety inspections.

Front Company

Having a front company allows for various types of cover for secret operations. For example no secret base is complete without minions. The minions could be disguised as miners and so would have a good believable cover for why they are coming and going from the base for whatever missions they have been tasked with. Diamond mines require security, so you have a very good reason for why you have armed guards around the mine.

Mines use a lot of equipment including some specialized equipment. So you have a very good reason why there are trailers moving unusual equipment in to set up your secret base with. Also you can tap into existing infrastructure without raising suspicions.

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    $\begingroup$ Or just grind the rock up and sell it as gravel. I was going to post an answer to that affect, but I think yours will do the trick. Make it a real business which could help fund the secret base. $\endgroup$
    – Greg Burghardt
    Commented yesterday
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    $\begingroup$ Of course, the downside is that you make a successful business that's worth a lot of money, and then plans for world domination get scrapped in the name of good old-fashioned profit-making. Capitalism ruins everything, doesn't it? $\endgroup$
    – Greg Burghardt
    Commented yesterday
  • $\begingroup$ Or start making alternate material cinderblocks, bricks, pavers, etc. Like gravel from Greg, you have a reason for noise/vibration (as long as no-one is trying to triangulate the noise source), you have your enemy pay for your base, have a reason for your logistics tail (trucks), and you can even make a giant impressive office building and showroom for investors to see how great your new building material is. But by all means build a tunnel to the nearby river and make a sub-pen too. If you discharge debris (welcome the lawsuits, but delay 'em) then no-one can see your subs coming up-river. $\endgroup$
    – Seth Robertson
    Commented yesterday
  • $\begingroup$ @GregBurghardt when I realized this technology could print diamonds the size of bricks my thoughts went to the TV Tropes: tvtropes.org/pmwiki/pmwiki.php/Main/CutLexLuthorACheck $\endgroup$
    – Anketam
    Commented 19 hours ago
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I recommend multiple strategies.

Use the compressed rubble as building material
If I am not mistaken, Stishivite could be a pretty good building material, especially when you have more than you need. The probably easiest way would be to have the Stishivite as bricks.
First of all, use it for everything that you need to build, e.g. inner walls of your base and so on.
I have doubt whether you can use the Stishivite bricks to stabilize your tunnels as brick walls are not that stable when it comes to extreme forces, concrete might be better.

Embed the tunnel dumps directly into the tunnels
When you dig a tunnel, make the tunnels bigger and embed the compressed rubble directly into the walls, floor and maybe the ceiling.
Or in other words, dump the compressed rubble where you dig, this way you omit the logistics to transport the rubble around.

Dig dumping tunnels for additional rubble and waste
Digging one big dump pit would be difficult, especially for the stability. Just dig some dead-end tunnels to dump the additional stuff.

  • Not everything you encounter during digging can be compressed into Stishivite
  • For the place where the secret base will reside, you probably want to dig a bigger cave. Here it might be difficult to embed all the rubble into the outer walls or use it as building material.
  • You can use those dumping tunnels also for any other waste
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    $\begingroup$ We're talking about building below-ground though, so I'd expect Seifertite (only minimally less dense) to perform decently as a building material. $\endgroup$
    – QuestionablePresence
    Commented yesterday
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    $\begingroup$ "Metastable" does not mean bad building material. Diamonds, glass, and aragonite shell material are metastable for example. $\endgroup$
    – Kevin Kostlan
    Commented yesterday
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    $\begingroup$ @QuestionablePresence (and to Kevin Kostlan) Thanks, I misinterpreted that it could easily break apart and revert into its prior components when stressed, but diamonds are a good example how stable metastable materials can be. I will revert that again. $\endgroup$
    – Thibe
    Commented yesterday
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Just bring the spoil out in a non-obvious manner, and dump it somewhere non-obvious - no need to keep it underground. To do so, build your lair underneath an active manufacturing or construction site. Trucks come in, trucks leave, but some are filled with spoil instead of manufacturing products. The spoil trucks deliver their cargo to a ship, which dumps it in the middle of the ocean. No one is ever aware that anything is leaving the site other than what they can see from the cover operation.

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Unfortunately, stishivite isn't stable at STP. It quickly degrades into alpha-quartz. Alpha-quartz might still make a decent building material, but it doesn't have the density advantage.

You should search for what they did to hide the excavated material (called "spoil") from NORAD. Most of it was shipped out by truck at night and used as fill for other projects. It's also rumored that they used underground rail spurs to transport it.

If the digging organization is well financed, I recommend trucking it to somewhere else and pretending that you're mining the material there.

Addendum I did a little digging based on Kevin's commentary. Based on This article, stishivite will stay stable for thousands of years at room temperature, as long as you don't get it wet. If you add water, it'll decompose in a period of days.

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  • $\begingroup$ Yes it is up to 500C if dry at ambient pressure and about 150C if wet at low-ish pressures: pubs.acs.org/doi/full/10.1021/acsomega.8b00484 $\endgroup$
    – Kevin Kostlan
    Commented yesterday
  • $\begingroup$ @KevinKostlan, That article states that it's metastable, not stable. Here's an example of what happens to meta-stable materials over time. youtube.com/watch?v=sXB83Heh3_c $\endgroup$
    – Robert Rapplean
    Commented 15 hours ago
  • $\begingroup$ If it lasts for "a moderate amount of time" near 500 C it probably lasts a very long time at 20C given how sensitive reactions are to temperature and the fact that heat drives the transformation forward. $\endgroup$
    – Kevin Kostlan
    Commented 8 hours ago
  • $\begingroup$ @KevinKostlan I'm sure that statement is correct, but we're talking about using something for a purpose that can cause structural collapse if even a small percentage of it destabilizes. This is a no-go for engineering purposes, even if you only get a small percentage of destabilization over fifty years. $\endgroup$
    – Robert Rapplean
    Commented 7 hours ago
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So, lets assume we have a near todays humanities capable foes watching from above. That gives them satellite ground resolution of 30 cm for detection.

Now lets assume, you also have the ground all mapped out. So you can store the molten down and compacted debris- in caverns and underground lake and rivers.

Finally, you could inject the debris into a sort of mud volcano or another natural phenomena that ejects material. Undersea avalanches, swamps and volcanos come to mind. Ideally, you place it in a plate tectonics https://en.wikipedia.org/wiki/Subduction zone and watch all your evidence being dragged down under. And if Mount Everrest lifts a meter more per year- thats gonna be hard to detect.

Another idea is - to hide it, not in the ground, but in the sky. You build a sort of railgun, hiding beneath the ocean waves, once it leaves the surveillance envelop you can fire your debris into the sky, to become part of some asteroid belt- or have a reentry, as just another asteroid.

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  • $\begingroup$ Railguns firing to orbit are not...subtle. Even with the nice (corroding) heat dump of the ocean. Sound and fury signifying...something to investigate with all speed. But, ignoring that, maybe with a little guidance package, you can turn that railgun into a fractional orbit bombardment system. $\endgroup$
    – Seth Robertson
    Commented yesterday
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I think your proposed approach is already about as good as you can get. Mine a tunnel 1.7 times the desired living radius (so, 3 times the cross-sectional area), compress the material as it is mined, in the boring machine, and use it to line the tunnels to a thickness of about 1/3rd the diameter of the tunnel. It'll look kinda cool, being a deep semitransparent layer. Indirect lighting that shines into the walls would look super cool.

Assuming modern technology, stealth requires:

  • Not being crazy loud. Some kind of advanced mining machine can handwave this away. It zaps the rock to powder and quietly scoops it away or something.
  • Not making vibrations detectable by sensitive seismometers, which can detect vibrations of just nanometers in amplitude, at accelerations of about 1/3 of a milligravity. Being under a city, or other source of irregular, noisy vibration, can reduce detectability.
  • Being deep enough under ground that it is not detectable using ground-penetrating radar. Being under wet clay and swamps will help reduce this problem. Dry sandstone is worst. The drier and looser the rock above, the deeper it can penetrate, to a couple hundred feet in the worst case.
  • Not moving any material away from the tunnels because otherwise gravimetric measurements could detect the moved material. As could satellite surveillance, etc.
  • Avoiding long rails or cables within the tunnels to avoid magnetometric detection.
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Borrow an idea from "Bill the Galactic Hero" https://scifi.stackexchange.com/questions/297841/story-involving-trash-disposal-and-mail/297842#297842 and just mail a box of "free product samples" (stishivite bricks or whatever) to a randomly selected person somewhere in the world.

The concept is, you foist the transport problem onto someone else and do it in such an obvious way that you cannot be suspected of hiding what you are doing. Being a stupid idiot, sure...

However...the joke's on you. Because of all of the tons of product samples you shipped out, at your cost the postal service improved their logistical skills and the physical strength of their employees. So when you launched your evil plan they swarmed your base, packed up all your minions, and literally shipped them off to jail.

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Convert the quartz into the colorless gas silane

Given a source of a hydrogen (such as the water in the air), you can convert your quartz into silane, which is colorless and lighter than air. So it will just float out of your base to the surface and no one will know (as long as you careful to not explode it)!

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  • $\begingroup$ Neat idea, but the sheer amount of pungent silane you would need to create and release... $\endgroup$
    – Kevin Kostlan
    Commented 7 hours ago

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