Dreamed this during a hot daytime nap I fell into whilst recovering from a hangover.
I was an astronaut visiting Neptune. We landed on the one seemingly constructed structure on the planet's ocean surface. It was a large cube with fractal-like subdivisions creating a grid of square tunnels throughout. It was covered in sea life, coral, algae, barnacles, etc. with little fish and rock pool creatures inhabiting the harsh corners of the structure. As me and my team swam through, we encountered the supposed intelligent inhabitants of Neptune. Giant fish shaped like cubes with softly rounded edges. These behemoths swam slowly through the hallways and it was clear that this structure, with its infrastructure focused on the cube fish, was a passing point, a train station, a liminal space. I looked out onto the open ocean and decided to return to earth without telling anyone about what we had found.