I have a mountain range that separates two places that certain people will want to go between fairly regularly. Right now they have to travel around or over a mountain range to do that.

I've been thinking about ideas of how to make traversal easier and quicker and one possibility that occurred to me was a cave(rn) system that could be entered on one side and eventually exited through the opposite side, similar to what Tolkien did with the Mines of Moria.

The difference here, however, is that Moria was excavated while I'm supposing this to be a natural formation. I don't know enough about geology to say whether or not this is absurd.

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    $\begingroup$ I grew up on the foothills of Sierra Bernia, in Alicante, Spain. It has a tunnel through it, it's not very long and you need to crawl, but it takes you from one side to the other. Here's a description of a hike that takes you through this tunnel: linkalicante.com/en/bernia-loop-hiking-trail $\endgroup$ Commented May 28, 2022 at 6:12
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    $\begingroup$ See en.wikipedia.org/wiki/List_of_longest_caves - but note that Karst caves (which were formed by underground water streams) can still be flooded occasionally, trapping spelunkers for days until the water has drained. This has happened in the Hölloch a few times, and in a few cases, new "side entrances" had been opened to rescue the trapped spelunkers (a rather small effort, practically negligible when compared to the construction of Moria). $\endgroup$
    – Klaws
    Commented May 28, 2022 at 9:46

2 Answers 2


This is definitely possible, but often such systems are not so easy to walk in and out. Please take a look at this cave in Abkhazia: https://inis.iaea.org/search/search.aspx?orig_q=RN:47073167 It has four known entrances on different heights, distance between them is several kilometers: http://www.rgo-speleo.ru/caves/hipsta/sneg_p_sm08.gif . Take a look at its entrance: https://v-georgia.com/wp-content/uploads/2015/06/0_eeecc_30312dba_orig.jpg – not an easy path, right?

There is also a smaller cave that has entrance and exit, that is available to the general public to visit: https://web.archive.org/web/20180106103509/http://www.abhaztur.com/upload/files/peshhera_2.jpg but it is not as long, probably less than a kilometer.

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    $\begingroup$ Do not forget subterranean rivers! Their wear on the rock can make such routes as well. The people might just be lucky with the caves. Maybe very little extra work was needed to make it a viable way of travelling. Or the difficulty makes for high prices for passage to use the system/manpower within. $\endgroup$
    – Trioxidane
    Commented May 27, 2022 at 9:46
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    $\begingroup$ @Trioxidane right, those processes are also called karst: en.wikipedia.org/wiki/Karst , Abkhazian caves are formed due to karst processes. $\endgroup$
    – vissi
    Commented May 27, 2022 at 9:55
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    $\begingroup$ Thank you, this sort of real life system is exactly what I was wondering about! $\endgroup$ Commented May 27, 2022 at 22:41

IMO It's improbable, but not impossible

Caves are formed for a variety of reasons, but the most common is erosion. So, is it possible to have a mountain made otherwise of basalt or bedrock to have a seam of sedimentary rock running through it that could be eroded into a mountain-traversing cave system?

Sure. There are no examples on Earth that I know of, but I can't see a reason why it's impossible.

Please note that such a condition would be naturally unstable. The seam would represent a weakness in the structure of a very heavy mountain. Add any kind of seismic events and that weakness would cause the cave to collapse.

But that's good for your story, isn't it? It means the cave system isn't entirely safe, and that's always good for drama!

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    $\begingroup$ Thank you for your thoughts! You are quite right, it does give an interesting dynamic to the plot. $\endgroup$ Commented May 27, 2022 at 22:40

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