There is a soft sci-fi RPG called Numenera, set on the Earth a billion years in the future, where multiple technologically-advanced civilizations have risen and fallen, and a new human civilization is rising among the ruins of the old. Not all of these civilizations were human, and some were in possession of technologies that included things like psychic powers, space flight, dimensional manipulation that allowed for travel to places like realms of pure sound, ubiquitous nanotechnology, large-scale planetary modification, and other similar things; while there's no magic as such, there's a very wide array of "magical" technologies at play.

In one of the adventures in the newest set of core rulebooks, the players explore a ruin called the Door Beneath The Ocean that is a giant, roughly circular "rock" structure located in a major mountain range whose material is laced with fibers of a plastic or composite that glows when things move nearby, and whose surface appears "whorled" with curved protrusions and "towers". Above the ruin is a massive, transparent sphere, of a similar size, full of water; it's now full of aquatic plant and animal life, but this may not have been true when the ruin was constructed. Winding through it are a number of maze-like corridors and roughly rectangular chambers, the smallest of which are over a hundred feet across in the long dimension, the largest approximately three hundred feet in both dimensions.

These chambers are usually filled with debris in the form of scrap metal, plastics and composites, and fragments of the stone the ruin is built out of; only a few of them still have functional structures within them. One has an archway that renders people who pass through it unconscious and grants them the ability to speak an ancient language, and another one that contains a permeable membrane over a pit full of preservative fluid and a gravity-manipulating device that activates to pull objects and people who pass near it into the fluid. Then there's one that contains a computer control system for the ruin's systems and an automated turret that defends it, and a fourth with the titular Door Beneath The Ocean: a transport tube that, when activated, will transport the user up a tube into the sphere of water above.

What purpose could such a ruin have served when it was constructed? The obvious answer is something like "giant water tower", but I'm sure the users of Stack Exchange can come up with something I didn't. The weirder the better, to be honest, but if you can back it up with references to actual science, and explain why and how it would fit the design of the ruin, that'd be good as well!

put on hold as primarily opinion-based by Gryphon, RonJohn, James Aug 10 at 14:07

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • Asking this here rather than the roleplaying site because the question's about the worldbuilding of an RPG world, not about playing an RPG or its rules. – nick012000 Aug 10 at 11:24
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    While this is interesting, it is also very opinion based. There are no criteria that would result in one answer being better than another. – Gryphon Aug 10 at 11:25
  • Your question is on the long side. While I think it is well written enough, perhaps think about what you really need from your first paragraph and see if you can perhaps introduce a couple of . here and there. I think your 3rd paragraph is basically just one sentence right now (and #2 2 as far as I can tell). Otherwise, do you have any criteria as to what you are looking for or perhaps more important what you are not looking for? – Raditz_35 Aug 10 at 11:38
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    "Beneath The Ocean" is a really poor place to try to pump water from. #1 It's far away (oceans are deep), #2 You've got to dive that deep to install the infrastructure (also really complicated and expensive to do). #3 Immense water pressure from the ocean above -- especially when combined with corrosion -- is going to cause water mixing. #4 Sea water is denser than fresh water. – RonJohn Aug 10 at 12:17
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    It was obviously a giant neutrino detector. – o.m. Aug 10 at 16:43

It could be a nuclear reactor. The glowing lights may be radioactive materials fused with the structure. This kind of radioactivity seems to have an euphoric effect to the players, but in the long run if they stay too long there they risk cancer and mutations. Did you ever made all your players bald and losing their teeths? Funny times.

It is an overthinking trap.

A rare safe for work moment im Oglaf Source: Oglaf. It is usually NSFW, so I am not including a link - you can find it by name on Google.

Seriously, people build big random things just because it's cool. Like the giant obelisks in Washington DC or in the Vatican. Perhaps the architect was going for a grandiose building like Saint Peter's Basilica, or the nazi Volkshalle. Or maybe there was something akin to the Guinness Book of World Records in the lost civilization and someone engineer was trying hard to make into it.

The thing about trying to understand lost civilizations is that, without a time machine, we can only ever guess what they were actually doing.

  • At least source the comic. This comic is from the fantastically NSFW webcomic Oglaf: – hairydresden Aug 10 at 13:12
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    @hairydresden Source link added – MichaelK Aug 10 at 13:26
  • Good comic but you know the site pretty well, try to help users improve questions before answering. – James Aug 10 at 14:13

It was a (secret / forbidden) scientifical facility where (secret / forbidden) marine animals where bred and studied.

The "Door Beneath The Ocean" is simply the feeding door where divers could pass into the "fish tank".To examine specimen, they where automaticly transported through the tubes into the preservative fluid.

All the other halls and chambers are what's left of labs and accomodation of the scientists. The language-teaching-archway was actually a defence mechanism that reads the thoughts of anyone passing through and alerts the scientists of traitors. That you can understand another language is just a side effect.

While I agree this is probably opinion based, I'll take a swing at it any way. It's a cistern. It's purpose it's to capture and store rainwater for the citizens to use for drinking.

You can check, but I believe this is close to the size of some of the largest cisterns from bronze age middle east cities. (Edit/correction: It is many time larger than the largest known cistern, the Basilica Cistern in Istanbul. My belief and $0.50 will bet you a really bad cup of coffee)

Yeah, this is a more technical way of saying "something like a giant water tower", so I guess this one will get downvoted pretty badly.

  • No cistern under the ocean. That wouldn't make sense. – RonJohn Aug 10 at 11:57
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    But was it under the ocean billions of years ago? – pojo-guy Aug 10 at 11:59
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    It's actually located in the the mountains. The "ocean" referenced is the half-mile sphere of water. I'll edit my question to be clearer. – nick012000 Aug 10 at 12:02
  • If you know it should be put on hold please refrain from answering and help the user address the problems in the question first. Check out How to ask for guidance. – James Aug 10 at 14:12
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    @Thierry Thank you, corrected. – pojo-guy Aug 10 at 16:04

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