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In a setting I'm working on, a creepy fungus-based ecosystem exists in a cave network below the Earth's surface.

I don't know how/if this is an answerable question, but -- how many miles (I don't need anything specific -- just an order of magnitude) of subterranean caves could exist (specifically under North America and the Pacific Ocean, if it matters) before it started to have a notable impacts on other systems (geological, water, whatever) that could lead modern scientists to discover there must be huge caverns down there?

There are small entrances to the surface world, but these would be a non-factor for the most part, as the caverns are home to a fungoid race who would kill intruders.

If there's no way to make this work realistically in a modern scientific framework, I'll just go high fantasy, but I'm curious about if there's anything realistic about the idea.

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    $\begingroup$ Define "huge caverns"? Huge caverns only rarely occur in caves at all. The largest cave ever found is 200m wide and 150m tall and had its ceiling collapse in two locations opening the cave to the sky. As spelunkers can tell you, caves are mostly for the claustrophobic, with open spaces seldom larger than a large room. If this were not the case, and such caverns were common, it would definitely have been discovered in the course of geological surveys. $\endgroup$
    – jdunlop
    Commented Nov 3, 2023 at 0:49
  • $\begingroup$ Thank you for pointing this out. Yeah, I think "huge" caverns as I envision them (think pulp-fiction style caverns large enough to fit jungles and megafauna) would probably be infeasible physically -- or at least get discovered quickly. As of now it seems like handwaving with some sort of magic BS is the best option here. $\endgroup$
    – BZawhi
    Commented Nov 3, 2023 at 1:16
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    $\begingroup$ If you are fine with a network of long tunnels without any major open spaces I think these could be enormous without anyone noticing them. $\endgroup$
    – quarague
    Commented Nov 3, 2023 at 1:27
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    $\begingroup$ You want fungus-filled caves under the Pacific Ocean. Just checking - are these caves supposed to have air pockets for the fungus to grow in, or are these water-filled caves with fungus that grows in seawater? $\endgroup$ Commented Nov 3, 2023 at 2:32
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    $\begingroup$ This question falls into my category of questions labeled, "how many angels can dance on the head of a pin? Answer: As many as wanting." There's no way to prove how many caves we don't know about. (Obligatory Men In Black III quote: "how the hell do I know what I don't know?") In other words, you might be straining at a gnat. Pick the number you want and move on with your story. Otherwise we need to know things like up to what depth below the surface? Water filled or not? Do lava tubes count? How structurally sound do they need to be? Etc. $\endgroup$
    – JBH
    Commented Nov 3, 2023 at 20:53

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There are caves everywhere that are completely unknown - or at least unmapped - by humans... miles and miles of them. However, caves are very rarely particularly large, usually no more than a few hundred meters across at most, and typically only a few metres across or less, and when they are located underwater, they rarely if ever are dry and not filled with water. A cave beneath the pacific ocean would realistically be filled with water, and salt water at that.

Completely underwater caves are often larger than dry caves, because the buoyancy and pressure of the water reduces the tendency of large caves to collapse.

I suspect that the OP wants large dry caves under the oceans in which to have their fungoid race live there. I would doubt that any caves that large could exist in reality even if flooded. It's probably best to make them a fantasy element, and drop a casual line from a scientist who says that he or she never thought that such a place could exist. Fantasy caves for a fantasy fungoid race would be fitting.

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  • $\begingroup$ "a few hundred meters across" and "a few meters across"? It can't exactly be both. $\endgroup$
    – Hearth
    Commented Nov 4, 2023 at 23:02
  • $\begingroup$ @Hearth no more than a few hundred metres across, and typically only a few metres across or less. In other words, usually very narrow, and only occasionally larger. $\endgroup$
    – Monty Wild
    Commented Nov 4, 2023 at 23:44
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In general we don’t have a lot of high-resolution data about things under the ground. The only ways to “see” underground features at any depth are by measuring tiny variations in the Earth’s gravitational field (gravimetry), or by setting off explosive charges and measuring the reflected shockwaves (seismic surveys).

Both of these are mainly done to search for new oil and gas fields. Seismic surveys have better resolution (tens of meters), but cover a relatively small area and are expensive to do. Gravimetry can cover a wide area – it can be done from satellites, though I don’t know if any useful scans of the whole planet have ever been done – but the resolution is even worse, and the data is complicated to interpret.

As I understand it, most places on Earth have never been surveyed in enough detail to detect even large cave systems. Even if the survey was done, it would be proprietary data, so no one would hear about it unless it looked like there was oil there, or if the data were unusual enough that someone could raise quite a lot of money to investigate. And the survey wouldn’t happen in the first place if oil companies didn’t think there was oil there, or if they couldn’t drill there because it was a populated area, for instance.

As you note, caves can be inferred in other ways. In the Mendip hills where I used to live, water is known to circulate underground (scientists can track this with marker isotopes and such), and there are lots of known caves, so it is reasonable to assume there are other, unknown caves. They could be huge and connected, or maybe there are thousands of tiny pockets; there’s no pressing reason for anyone to spend huge amounts to find out. If there weren’t millions of people using water in the area, we wouldn’t even know this much.

One thing I would point out, though, is that if there was weird life in those hidden caves, that might be detected, and attract investigation. Unless the cave biome was completely sealed, and I don’t know if that’s feasible, the water coming out of it would contain spores and metabolites and stuff, so you’d have unusual ecologies around springs at the surface. (But this could still go undetected in a remote area).

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  • $\begingroup$ I'd like to point that most wildlife in deep caves are unusual/weird by nature. The most well-described species tend to be whiter than paper and lack sight since there's little to no light. $\endgroup$ Commented Nov 3, 2023 at 17:17
  • $\begingroup$ but the pacific coast of north america is one of the most seismically mapped area on earth, at least the area near the san andreas. $\endgroup$
    – John
    Commented Nov 4, 2023 at 11:41
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So it would seem that the only means the surface-dwellers have of detecting these caves is if they caused (a) geological changes (eg: sink-holes) or (b) unexplained reduction in surface water supplies.

Regarding (a):
I would hazard a guess that the maximum sub-terranean cave volume that can be accommodated without impacting the earth's surface will be determined by the factors that civil engineers and mining engineers are familiar with. Full disclosure: I am not either of these.

My guess is that these factors will include:

  1. Cave volume vs depth below surface. At a given depth, there will be a maximum permitted cave volume before it begins to impact the surface. I am informed that this value increases with depth.
  2. The geological profile of the cave location. Locations with dense igneous rock will support bigger caves than those with sedimentary rock, for example. If the cave system covers a large area then the rock will also vary across the extent of the cave formation.

Regarding (b):
Again, mining and civil engineers may be the best sources for opinions on this, and since this is regards to water perhaps add environmental scientists/engineers to that list.

If the changes to the surface water volumes was sudden, then the surface dwellers would notice and possibly investigate. However, their propensity to investigate in a scientific manner will depend on their technological capability, and the political and cultural circumstances - for example, a religious society whose leaders explain the sudden loss of water as "an act of God punishing a sinful people" would be less likely to pursue and fund an effective scientific investigation. This threshold for a detectable rate of change of water supply is therefore highly dependent upon the social construct of your fantasy world.

Existing Cave Complexes
Here is just one example of an existing cave system that is very large: the Cappadocia region of Turkey. At least 200 substantial cave systems have been found, many of which can accommodate a human population numbering in the thousands.

https://www.easternturkeytour.org/underground-cappadocia-land-of-mystery/

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~50 million miles.

The Mammoth Cave in Kentucky is the world's largest dry cave. It's 700+ km long over an area of 214 km^2. This gives a ratio of 3.5 km/sq.km, allowing about 80 million km or 50 million miles of caves to potentially exist under North America.

Such a cave system would not go completely undiscovered with modern technology, since underground mapping by various methods is being done all the time in search for oil and other resources. For a maximum size that might go unnoticed, pick the largest area you think no one would ever look for oil, gas or ore under, and plan about 5 miles of cave per square mile.

Maybe there could be an explanation, e.g. if some piece of land was left to the natives. Say, an alt-history, where Alaska never sold, conquered, or developed, and was relegated to the Inuit as a compromise.

All caves below the ocean floor are fully flooded. There is no realistic mechanism or explanation that would allow naturally formed undersea caves to stay dry. Even caves under dry land tend to fill with water during the rain, and only stay dry if they are above sea level and able to eventually drain.

There are small pockets of air in some underwater caves, and a cave can stay partially dry under shallow and scattered bodies of water, such as pools, shallow lakes, and rivers. Large bodies of waters, such as lakes, inevitably seep into the cave below eventually.

A large well-known dry underwater cave used to exist under Lake Peigneur, although the cave was man-made. There are similar known incidents. In all cases, the water bodies above were shallow.

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