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There is underground vault built to last a few million years. The contents of this vault are all durable things, such as things carved in stone. Inside this vault is a door to an inner room. While the door is safe from "the elements", the vault will undergo numerous tectonic shocks and thermal changes over the years.

The vault is underground, so it doesn't need to survive weather, but it does need to survive whatever changes will happen over the intervening millennia, such as tectonic motion, the planet changing temperature, changes to groundwater, degradation of surrounding rock, etc. My assumption is that it's impossible to design a hinge that would survive such treatment.

How is the interior door constructed such that it stays closed, thus keeping the inner room sealed, but can still be opened after all this time with minimal effort.

The people opening the door have access to a reasonable assortment of portable tools of near-future tech level but no heavy machinery. They are willing to spend a few hours opening the door.

Clarifications:

  • The makers of the vault had access to future tech (modern technology plus several hundred years).

  • The makers of the vault designed the vault to last at least a million years.

  • The makers of the vault were not particularly concerned with looters. The vault is located such that it is only accessible by the right people.

  • The vault's location is important, so it can't be in a place with no weather like the moon.

  • The door should be weather-proof though airtight isn't necessary. The goal is to keep the contents of the room in prime condition. The contents themselves are fairly durable, but keeping water out would be good.

  • The door does not need to be resealable once opened. It can be an open once door.

  • It should be fairly obvious that it is meant to be opened.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jul 12, 2022 at 18:52

15 Answers 15

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Sand door.

There are two rooms, 'labelled vault' and 'closed room'. The closed room contains a heart with the word 'love' on it. Both rooms have a hole that connects to a single large pit of sand.

The door is a hole in the floor. The hole is full of sand. If you dig it out, it connects to another hole in the floor of the adjacent room.

The door is made of sand. The sand will keep bugs from getting into your closed room. Sand will stay sand for a million years. Cats may add cat gifts to the top layer of sand which will not impair its function as a door. If the room gets flooded sand will not float away. It will not corrode. You could open this door with your bare hands but you might want to use a scoop because of the cat gift issue.


Edit Assuming the vault contains the "stuff" to be preserved and the entrance hole is filled with sand from the other room. Allowing for the angle of repose of the sand, the backfilled hole will look something like the picture below.

The same as the previous image, except the sand in the vault room hole is at an angle.

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    $\begingroup$ In a million years, wouldn't the sand turn to rock if undisturbed? $\endgroup$
    – DWKraus
    Jul 11, 2022 at 5:45
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    $\begingroup$ @Willk: I love the idea. For added hijinks, cover the entire floor of the "closed room" in 20cm or 30cm of sand, with a "pit" 50 cm or 60 cm deep on the side. It'll take a curious or really luck soul to realize that there's a tunnel under that sand in one specific spot. $\endgroup$ Jul 11, 2022 at 14:57
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    $\begingroup$ I'd also elevate the floor of the inner room above the level of the vault's access, so if there would be a flood from above the water would drain from the vault via this sand "door" and its contents will remain unaffected by water, or not reach the inner room should the leak be in the outer room. And about closing the door first time - just fill the vault with sand bags and place a trigger that would make those bags empty into that side of the "door", then fill the door from the outside, trigger the inside drop and pull out the mechanism by the wire. $\endgroup$
    – Vesper
    Jul 12, 2022 at 12:58
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    $\begingroup$ Just one question about your drawing, what's love got to do got to do with it? $\endgroup$
    – Glen Yates
    Jul 12, 2022 at 14:41
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    $\begingroup$ @GlenYates youtube.com/watch?v=PT1ySOeS7oY $\endgroup$
    – Willk
    Jul 12, 2022 at 20:09
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If the vault will "undergo numerous tectonic and thermal shocks over the years", you don't want to fully seal it. No material will keep its integrity over those shocks. Metals will flow. Even rocks will crack or warp. When thinking in terms of millions of years, climates radically change, and erosion will increase or bury the access even more.

It is better to design the system to breathe and move with the surrounding stones so that it doesn't flood with ground water, doesn't warp and either pop open or completely jam tight, and doesn't have atoms migrating between parts. The caves where we are finding ancient hominid bones properly preserved are deep, dry, and have a structure allowing them to breathe but still block off most atmosphere changes and make it difficult to get back there.

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    $\begingroup$ What exactly is the 'door' here? $\endgroup$
    – Joachim
    Jul 11, 2022 at 18:49
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Solid Gold:

If your vault itself doesn't get buried, crushed, and flooded, it would likely get forgotten. Minimal effort is also a complicated thing to define. But I'll guess this is either meant to preserve knowledge for a future society (in which case they need to find and recognize it) or else preserve information for semi-immortal/time traveling/relativistic travelers who are planning to come back. The terrain will have changed radically, and I think the biggest trick will be getting people to find the place (and possibly making them think to dig there). So let's make a plug of solid gold.

Even primitive people valued gold as a material. Gold doesn't rust or tarnish. Its density will be measurably different and detectable by a reasonable advanced technology if buried. If not buried, people will readily recognize it as being something extraordinary. Even if the door is warped or crushed by geological forces, stone-age people could and would dig it out with the tools they had. Due to the low melting point, advanced tech could liquify the door without endangering the stone materials within. And if the gold door is a plug lubricated by a mineral like talc, it might be able to slide out of place with the effort of anyone able to move the weight. Gold is heavy, so even just resting in a plug it should likely stay in place.

Gold also makes a good material to line your vault if you hope to keep it sealed (no guarantees). Scrolls of gold could last a million years if properly stored.

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    $\begingroup$ How would you prevent the door itself be mined for its gold ? When you can easily open this door, you can also chip off gold from it. $\endgroup$
    – Goodies
    Jul 9, 2022 at 14:50
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    $\begingroup$ @Goodies If you wanted people in the future to discover the door, them mining it would be a DESIRABLE outcome. They otherwise might ignore it and not discover the contents. $\endgroup$
    – DWKraus
    Jul 9, 2022 at 16:16
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    $\begingroup$ The issue is that this guarantees that whoever finds the vault first will mine the door and take it, leaving the contents to the elements (or probably to be looted as well). Now, that might be in a million years, like the OP wanted, but it might be in a hundred years, or two years. At the very least, a vault made of stone or whatever probably won't be carved up by the first peasant to come across it, so I'd personally expect it to last a lot longer. $\endgroup$ Jul 9, 2022 at 22:37
  • $\begingroup$ @VentifactsandYardangs Burying radioactive goodies inside the stone and gold door could keep it safe for a few tthousand years $\endgroup$
    – DKNguyen
    Jul 10, 2022 at 17:59
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    $\begingroup$ @SeanOConnor I intended no moving parts, just a big gold plug. Easy to detect, attractive to future civilizations (if that is the desired effect), All one piece. Physically and chemically separate from the surrounding rock. Possibly with a loop to secure a winch to make it easy to move (if you can handle the weight). $\endgroup$
    – DWKraus
    Jul 12, 2022 at 2:10
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By maintaining it

With future tech involved we can assume they have access to robust automation and maintenance drones. Knowing this we can have weekly checks and maintenance of the door in question.

The requirements are sort of easy. Enough materials for energy storage and maintenance. Materials for building further power plants or mining/manufacturing/recycling for unexpected events. Robust software and hardware that, with maintenance and redundancy, can continue for millenia. Maintenance means it can tear down and rebuild any material or system, replacing it with a fresh new version. As any system is also redundant it can safely be rebuild.

Together with geological power from deep in the Earth, as well as the options to maintain or rebuild it in case of major changes over the millenia you have all the power and material to keep it running for those millenia.

The door(s) can be maintained and rebuild to your desire. Any unexpected nuclear explosions, major asteroid impacts, supervulcano or indigenous people digging it up and using it as a holy worship can all be reacted to by the maintenance drones and computers. This way you have much less chance of the door failing to be opened at the right time. It'll run as smooth as the day it was build. Possibly even better.

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  • $\begingroup$ How do you make software that runs for a million years? $\endgroup$
    – Zags
    Jul 12, 2022 at 17:26
  • $\begingroup$ @Zags with current and future technologies. First of all you want the stored data that is used for the software to be as stable as possible. I read some (experimental) crystalline hard drives that could potentially be used for millennia. Next is correction. There's methods to detect faulty or unintentional changing code. Last is comparison. If you have stored it several times you can compare them every once in a while and weed out faults. Same counts for the actual processing. Last is the maintenance and recycling. You keep the several computers and storage new by rebuilding/maintaining them. $\endgroup$
    – Trioxidane
    Jul 13, 2022 at 7:50
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Inspired by Douglas Adams/Terry Pratchett

Build any kind of door and provide an eternal* guarantee against it being opened.

You can be certain that it will open easily after a million years.

*For legal purposes, "eternal" is defined as 999,999 years and 364 days.

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    $\begingroup$ For comedy purposes this works just fine, but the OP's requirements were in terms of engineering. So: I'm upvoting then voting to close as low-quality. (Human response + typical bureaucratic one, Douglas would have approved). $\endgroup$ Jul 11, 2022 at 18:01
  • $\begingroup$ I'm just upvoting. I'm trying not to take myself to seriously today, and I loved the "n-1" guarantee. $\endgroup$
    – JBH
    Jul 11, 2022 at 22:21
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Q: "How would you make a door that could still be opened a million years later? The makers of the vault had access to future tech (modern technology plus several hundred years)."

Petrified wood

The science of geology forecast

Doors are supposed to be made of wood. If you drop thousands of tons of material on the door, petrify it and shape it, and put it upright in the far future...

The story is set in a quite distant future. In the next century, computers may be able to predict millions of years of geological events. The door is placed at a certain spot in the vicinity of volcanic activity.. In the next 320.000 years, the door will get covered by volcanic ash and rock, petrify. Then, a period of 500.000 years follows with convection in the crust, the petrified door being lifted upright, along with the hidden cavity containing the treasures. Now, 180.000 years of erosion will follow, exposing the door just in time for our primitive followup civilization to find it. Their presence was predicted as well, of course.. opening the door won't be easy, but doable. At a certain point in time, the door will collapse when it is not opened.

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Geological change is only a threat if you build in a bad spot

While 1 million years is enough time to see significant changes in some parts of the world, some cave systems are over 300 million years old because they exist in geologically inactive regions. Your future tech people will know about geology and be able to pick an ideal location to place thier bunker where they will know to expect at least 1 million years of geological stability.

Make the door out of natural minerals

With the stress of geological change out of the way, your next biggest enemy will be chemical changes. Generally speaking, the stuff we already find in the ground like quartz and granite are already elements lumped together into thier most stable possible forms after billions of years of chemistry have already had its way with them; so, if a quartz crystal can last for a million years; so, can a quartz door as long as it is adequately sheltered from rain and wind.

The stuff that does not last is the stuff we make ourselves. When we melt down iron oxides to get pure iron for example, that iron will for the rest of time be looking for oxygen to bind with to get back to it's more stable iron oxide form, but rust will stay rust for a very very long time.

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  • $\begingroup$ Sidenote: iron is found as rust only since the Great Oxidation. Before that oxygen available for binding was rare. Granted, such changes are so slow they are not a problem for a mere million years. $\endgroup$
    – spectras
    Jul 12, 2022 at 3:50
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A Really Big Rock*

Take advantage of gravity by just putting a big rock over the tunnel to your vault:

Big Rock

When future-people come to open it they can dig it partially out (I just put handtools in the picture but use your imagination) then push it down the the hill.

Or alternatively a "door [that] does not need to be resealable once opened" sounds to be in many words "a door that can be blown up":

KaBoom

(with a carefully controlled blast of course)

*Open to material suggestions

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    $\begingroup$ Welcome &#0x263A. I can't help but notice that the illustration has a certain likeness to your avatar. Makes me think, it's about time I answered a question about prehistoric birds. $\endgroup$ Jul 11, 2022 at 21:50
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Rubber Seal: tear along corner.

If the door only has to be opened once then the simplest solution is to construct a door which will only open once :p. Instead of a door just have a thinner section of wall with instructions to "break here".

Borrowing from an above answer, just make the door a slab of solid gold of high purity (that makes it soft). Ergo your archeologists wont have any issues digging through the slab, only issues with getting permission to open a closed archeological site - unless someone just steals the door, then problem solved.

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  • $\begingroup$ I like the approach of creating a "deconstuctable wall". The vault would most probably be carved out from granite or similar, so the walls would otherwise be solid. If the bricks are small enough, they can be removed by hand tools one-by-one, especially if no mortar is used. The only real problem is that seismic activity could shake loose some bricks. Actually, using granite lego bricks could solve that problem, they would need to be real precise, but future tech FTW! $\endgroup$
    – P.Péter
    Jul 11, 2022 at 13:09
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There are a couple different angles to answer this one... but I'm going with a crypt or "crypt door". There is an inner shutter mostly made with sheet metal and then sealed with glue/caulk.They have drain holes which air can get in and then an outer shutter which is made of marble,granite,etc.

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Option 1: Cheat. The door is a force field that expands into the available space, so if the dorway deforms the field changes to match.

It can be easily opened simply by turning it off.

Option 2, it's not a door, it's a physical seal. Maybe the equivelant of sticking a sheet of flexible rubber over the door. It's opened by puncturing it with a sharp object.

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    $\begingroup$ If it's a force field, how do you power it for a million years? That seems like a harder engineering problem $\endgroup$
    – Zags
    Jul 11, 2022 at 2:08
  • $\begingroup$ The OP mentioned tectonic activity, so maybe geothermal power? $\endgroup$ Jul 12, 2022 at 12:21
  • $\begingroup$ @AaarghZombies You can get energy by the flow of heat, not by the heat itself, to get energy from geothermal power you would need an access to a heat sink that will remain stable for a long time. $\endgroup$
    – FluidCode
    Jul 12, 2022 at 12:24
  • $\begingroup$ The OP states that the area is tectonically active, so it's reasonable to suggest that whoever built the vault could have drilled down into the ground and used the heat to create steam to power a turbine, or that they could have used some kind of science fiction handwave to extract electromagnetic energy created by the movement of rocks. It's up to the OP really as it's their fictional universe. $\endgroup$ Jul 12, 2022 at 18:16
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Interlocking bricks

First. If you don't know whether those who will enter the first chamber have the right tools you could leave there a couple of sledgehammers, if they are made with the right material and they are treated against corrosion they can resist a long time.

The door could be made by interlocking bricks with several key bricks placed is different directions. In this way if the whole place is tilted by tectonic movements a couple of bricks may fall off, but the wall will still stand. Just to stay on the safe side the wall can be made of several layers. It will take a long time to open it, but the future visitors seeing the bricks coming out one by one will keep working.

On one wall of the first chamber there could be the a carving with the instructions to remove the key bricks and open the wall. If the deformations pressed the bricks together the future visitors will have to work harder with the sledgehammers, not really a minimal effort, but still doable.

Final note, the bricks will be made of incompatible materials and mixed together, is this way the contact surfaces should not fuse together over time.

Dirty alternative: eutectic alloy

There are many alloys with low melting point. A tin and zinc alloy would melt at temperatures close to 200 degrees Celsius, that is high enough to resist geothermal stress and low enough to be melted with a wood fire. It would make a simple door with a tedious, but easy, opening mechanism. To avoid damaging the content of the vault the door could lead to a long corridor.

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    $\begingroup$ "the door could lead to a long corridor" - just elevate the floor of the inner chamber. A small ladder, 2-3 steps, is all that you need to protect it from the "door"'s meltdown when it'll happen. $\endgroup$
    – Vesper
    Jul 12, 2022 at 14:47
  • $\begingroup$ @Vesper You forgot smoke and fumes. You forgot that geologic movements might tilt the ground. $\endgroup$
    – FluidCode
    Jul 13, 2022 at 12:01
  • $\begingroup$ In the current settings, scrap fumes and smoke, they said to not be able to damage the vault contents. Also should there be smoke/fumes, the inner temperature should then rise and the door might melt, this however violates the declared "door is protected from "the elements"" condition. About tilting - while true, tilting down 45 degrees is highly unlikely without actually breaking the vault, and this would also break some of the other door constructions. $\endgroup$
    – Vesper
    Jul 14, 2022 at 6:39
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Woven asbestos and silicon jelly

Asbestos fibers are stable as minerals for geological durations. Silicon jelly is a broad class of substances which should be stable against water and oxygen. If a thick laminate of multiple asbestos layers is thoroughly impregnated with the silicone, capillary action ought to prevent it from flowing out entirely even over long periods. (Not that I can cite anyone who has tested this)

To make the door, you attach several overlapping layers of this material: some follow the surface of the inside hallway for some distance inward and are fastened at multiple points there; some conform to the external hallway; some are firmly clamped in one or more gaps between the structural elements used to make the inner vault. The layers may be sewn together in places with more asbestos and firmly pressed together with more silicon to keep them together for a better seal against water.

Opening the door requires something to cut with ("heavy dolphin-nosed tin snips" were mentioned in a search result). Please remember to bring along a decent mask, not a neck gaiter.

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Coat the interior and exterior walls with a thin layer of diamond to provide a maintenance free protective coating. The door, also diamond coated, could be a simple circular manhole style cover which fits perfectly into the opening. The vault is internally over-pressurized, which keeps the door in place from the inside and keeps out water. Merely pushing on the door would be insufficient to overcome the internal pressure, it would require a diamond drill bit to drill through the door or walls and release the internal pressure. (1 atm = 14.6959 psi, thus 10 atm = 146.959 psi) Once the pressure equalized, it just would take someone strong enough to push the door out of the way in order to gain entry. Provided there are no leaks, such a system should last quite a while. Don’t get me wrong, diamonds (a.k.a. carbon) do/does have a half-life, but it tends to be in the billion year range so there is plenty of time. We can currently make diamonds, so future-tech should be able to manage coating things in diamond.

Another benefit of diamond coating is that it would protect the surface from rusting/tarnishing/degradation. A silver (Ag) door would remain bright and reflective as long as the coating remained intact. You would still need to take internal material half-life into account, but would prevent oxidation and other chemical interactions/reactions. Heck, if you wanted to prevent access prior to a certain year range, you could make the interior of the walls and door out of a radioactive material with a half-life targeted to your “safe range”. Anyone approaching the vault before the material has decayed to a safe level will receive a dose of radiation. Aluminum-26 has a half-life of around 717,000 years, and produces both gamma rays and x-rays. It degrades into magnesium-26, which is stable. After 1.4 million years, the radioactivity will have decreased to a quarter of its original strength. At 2.1 million years, it will be at an eighth, and so on. If this range is too long, you can look for an isotope which fits. Just make sure the radiation emitted is sufficiently harmful and not blocked by something like skin or clothing. Not all radiation is equal.

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Making a door that will last a million years is easy. A thick rectangular rock slab sitting on top of and extruding into a vertical rectangular hole will last. An igneous rock like granite is preferable since they are less permeable to water, making them resistant to weathering.

enter image description here

Finding the door will be a much greater challenge. Typical positioning systems will no longer be valid in a million years. Satellites will have died or changed orbits, the earth's magnetic poles will have shifted, the constellations will have changed, continents will have drifted and risen and fallen, sea levels will have changed giving continents a different shape. Even if the original coordinates are known, the original coordinate reference system is now useless. Compounding the problem is the potential for the door to be buried in sediment from floods.

What knowledge and resources do the finders of the vault have? Do they have current and original satellite imagery? Can they do continental drift calculations? Even with those, there will be considerable uncertainty in the location. Intensive search will be required to find the potentially buried or destroyed vault. Perhaps your best bet is to place the vault in a large and well known cave system as the cave's internal structure will change much less than the earth's surface.

Tectonic movement happens at such large scales it is of no concern to a structure. Your structure doesn't care if your whole site moves 100 miles east. What would matter is differential movement across a structure (part of the structure moves 2 inches east while another part moves 2 inches west). The earth's subsoil temperature is stable and rock doesn't mind temperate changes as long as water is not freezing and thawing.

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