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!