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  • What is the oldest a still extant cave on Earth could be?
  • What conditions could encourage the longevity of a cave?

It seems that the oldest known caves, the Jenolan Caves in Australia, formed in the Carboniferous period.

  • How plausible would a cave from the Ordovician or even Precambrian be, considering that not even continents are stable over these time periods?

  • If these caves were designed, excavated, and reinforced back then by another civilization with modern technology, could they be engineered to last longer?

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    $\begingroup$ I suggest you split those questions, one about the possiblity of the caves, that gives us a timeline to work with and then another dealing with the artefacts. $\endgroup$ – Mormacil Mar 21 '17 at 16:33
  • $\begingroup$ I've split your question up into bullet points. It seems to me that the first three could be answered under the banner of What is the oldest cave I could have? however the other two might belong in a second follow up question. This way you can get concise but detailed answers on both topics rather than several pages worth for the single question. $\endgroup$ – Lio Elbammalf Mar 21 '17 at 17:06
  • $\begingroup$ Thanks for the edits. I removed the last question about artifacts, but kept all the others that were relevant to cave longevity. $\endgroup$ – tinydoctor Mar 21 '17 at 17:20
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    $\begingroup$ Are you specifically asking about caves on Earth (implied, but not stated)? In the Solar System? On an Earth-like planet? Please provide a bit more detail in the question to limit the scope to what you are actually interested in. You've already got one answer about caves on the Moon. After all, this is Worldbuilding. We could answer that some artificial planet was created prior to the existence of the Solar System, just to showcase the cave (although would usually involve technology beyond what would normally be considered "modern"). $\endgroup$ – Makyen Mar 21 '17 at 21:05
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    $\begingroup$ Note that caves can form as a result of lava flows, so some could have formed very early in Earth's history. $\endgroup$ – Hot Licks Mar 22 '17 at 0:26
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How to make a limestone cave

The Jenolan caves in Australia are reputedly 340 million years old. Those caves are in limestone deposits that were formed on the bottom of a shallow sea in the Silurian or earlier.

Limestone is the most likely substance for these caves to form in. But limestone isn't really a natural geological rock, it is formed by the crushed shells of marine invertebrates. There can be non-biological limestone, but it is much rarer, and probably nowhere pure enough to form limestone caves.

Limestone comes primarily from corals and foraminifera. Both these species developed in the Cambrian, but did not become common until the Ordovician. I can't find good data on how long it takes Limestone to form from these invertebrates, but it is millions if not tens of millions of years.

Assuming a few millions of years of depositing marine shells, then a few million more years of limestone formation, then this continental shelf (limestone doesn't form in deep water under high pressure) has to become dry land, then that dry land has to get enough rain to expand a cave system...you are probably looking at Silurian era for the earliest limestone caves.

How to get a cave to survive

The problem with limestone caves is once they start eroding, they are likely to keep eroding. The Jenolan caves are conveniently located a drier part of the Earth that has always been relatively dry. Australia has never been too polar or to equatorial over the last 400 million years or so. Also, it has never been underwater either. The only other part of the world that I can think of that meets both these criteria is southern Africa.

Cratons are probably not possible for natural caves

I don't think a craton could potentially have many caves. First off, they have never been underwater, so they were never able to get a layer of limestone on them. Secondly, I don't know of any other materials components of a craton that would make caves like limestone.

Incidentally, this will then rule out southern Africa, since most of that sits on the Kaapvaal craton. Maybe caves would be possible in the sedimentary fill between the Congo and Kaapvaal craton (roughly Angola/Zambia)? I don't know much about the geology of the area, and there isn't much information available online.

However, if we are talking artifical caves, then the safest place to put one would be in the heart of a 3 billion year old craton. Cratons this old include the Kaapvaal in Africa, Dharwar in south India (partially covered by the Deccan Traps), Pilbara in western Australia, Superior under Minnesota and Manitoba, Slave in Canada's Northwest Territory, and Sarmatian under the Ukraine.

The Witwatersrand formation is an igneous plug in the Kaapvaal craton that formed 2.7 billion years ago and is pretty much still the same today. An artificial cave there could have probably survived all this time.

Conclusion

Based on the likely cave forming processes, the oldest caves would be Silurian in origin, and would likely be situated in Australia or southern Africa. Since the Jenolan caves meet these requirements, it sounds like they are about as old as caves can get.

For artificial caves, the possibilities are more significant, since there are igneous provinces on top of cratons that are more or less unchanged for billions of years.

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    $\begingroup$ I've been in caves in igneous rock. $\endgroup$ – Joshua Mar 21 '17 at 19:28
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    $\begingroup$ @Joshua How old are they? If you are talking about a million year old lava tube, that isn't particularly relevant to this question. The kinds of areas that have lots of volcanic activity also tend to have lots of earthquakes and colliding plates, not great for cave survival. $\endgroup$ – kingledion Mar 21 '17 at 19:56
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    $\begingroup$ They're a few million years old now, but unless a glacier wipes out the substrate they will last a long time. A similar construction in a desert region (and this is plausible--that I didn't find any near San Rafael Reef doesn't mean there aren't any) would last for eons. $\endgroup$ – Joshua Mar 21 '17 at 20:02
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Do your caves need to be on earth? There are lava tubes on the moon. Lava tubes are a type of cave. This article estimates those tubes to be 3 billion+ years old.

http://www.asi.org/adb/06/09/03/02/100/12-questions.html

These tubes are bigger than terrestrial tubes, because of the low gravity. There has been a lot of enthusiasm as regards moon colonists moving in and pressurizing them.Here is a picture from BBC.com of Philadelphia in a lava tube.

enter image description here

If you are envisioning /another civilization/ Mountains of Madness type scenario maybe you could have it on the moon. Probably a good idea to check for that stuff before you send Philadelphia.

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    $\begingroup$ Judging by the size key, that must just be downtown Philly. $\endgroup$ – Willk Mar 21 '17 at 20:16
  • $\begingroup$ Well, it does say "approx" $\endgroup$ – Steve-O Mar 22 '17 at 14:30
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It's not impossible to have a cave from the Hadean since you have the early formation of the cratons of contents which are stable over long geologic timescales. Just really unlikely and to my knowledge, there are none that have been found. Such caves would have been formed by lava rather than the action of water and would contain a geologic record specific to that time. Such caves would have to be closed off from the environment for the majority of their life because over geologic time they would be filled in.

As for constructing an artificial cave, barring building it in the craton of a content and then sealing it somehow, there is nothing that could be done to allow it to last over significant geologic time scales.

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