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One of the ideas I've had for a story is to have million year old subterranean buildings/cities/temples which have younger architecture built on top of them, resulting in a "dungeon" which would be an archaeologist's dream.

My question is how deep can this type of structure be before it just collapses under its own weight?

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  • $\begingroup$ The two questions in this post is probably too much. The one question is about caves, and the other about architecture. I removed the architecture part to make sure this questions is not 'too broad' for this site. You can ask that question as a followup to this one. $\endgroup$ – kingledion Mar 22 '17 at 23:50
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    $\begingroup$ I saw this question and instantly thought of this video by Lindybeige. Might be helpful for your followup. $\endgroup$ – JBeck Mar 23 '17 at 2:22
  • $\begingroup$ lol, this video is what prompted me to ask question, I'd had the idea but hadn't quite realized what problems it might entail. $\endgroup$ – Eloc Mar 23 '17 at 3:18
  • $\begingroup$ how would i do a follow up question? just make it a seperate Q or is their some sort of thing? $\endgroup$ – Eloc Mar 23 '17 at 23:11
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The deepest caves that we know of are the Krubera Caves in Georgia (the country). These have been explored as far down as ~2200m deep. There are about 13km of explored tunnels, but none of the spaces are very large, nothing that would support a city.

The world's longest cave is the Mammoth Cave system in Kentucky (the state). These cover ~213 km$^2$ and have 652 km of surveyed cave tunnels.

The largest single cave room in the world is the Sarawak Chamber in Gua Nasib Bagus in Malaysia on the island of Borneo. This room is rougly 600m by 400m and has surface area of 0.16 km$^2$. That isn't super big, but could support a city of 10,000 or so with really packed conditions. There is an average of 58m of overhead in the Sarawak Chamber, so plenty of room for building up.

So those are the rough limits for a natural cave. With a little human architecture, and Roman-level engineering skills (aka the arch and concrete), it would probably be possible to make a series of rooms the size of the Sarawak Room covering an area the size of Mammoth caves. Think of them as neighborhoods of a few thousand people, each joined by short tunnels to the other part of the underground city.

As far as the catacombs go, 58 meters gives plenty of room for a lower level of ruined buildings, and an upper layer of active city or neighborhood on top of that. Since it is dark, there isn't really a lot of advantage in 'open sky' so no reason not to build to the ceiling.

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