Picture a landscape: a body of water, full of life and chatter, surrounded by plants, and inanimate nature in the background. The water represents vitality, creativity, intelligence. The mountains are far away. Plant species, capable of very basic response to stimuli, occupy the area immediately next to the pool. The animals, on the other hand, delve into it. While dogs barely get their feet wet, most humans are in water up to their knees. The scholars and artists stand in the water deep to their waist. Most prominent of them, to the chest. The great, era-defining geniuses have their heads submerged.
Each civilization takes a journey from the outskirts into the deep. Humans achieved remarkably much in mere thousands of years, but that time was but a blink of an eye in the cosmic scale. Delve into the water and discover the wonder humanity might achieve. See the alien utopias, their medical, cognitive, and wartime inventions, the unboundedness of their aspirations, depth, and infinitude of their entertainment. The complexity of their behavior, the purity of their emotion, the beauty of their art. What can the next ten thousand years of scientific progress bring? Next hundred thousand years of building new cities and colonizing new planets?
The deeper into the water, the stranger and more wonderful its dwellers become. Whereas variations, both obvious and counterintuitive, of bipedal humanoid form in a somewhat individualist society based on a type of market economy dominate the upper strata, the deeper you go, the more peculiar the aliens become. Silicon brains in engineered bodies are the majority. They also boast quite a diversity, with both the intelligence architectures and their physical bodies covering a vast space of possible configurations. Some other non-carbon and non-silicon-based lifeforms are scattered across—floating benign figures, friendly reaching forward with tentacles. There are sentient clouds, fluids, creatures occupying both a limited or an (artificially) extended number of dimensions. While the first few leaps downward lead to an enormous increase in diversity and abundance, each meter deeper than that goes with a gradual decrease in both.
As the pressure rises and becomes crushing, the light gives way to the overwhelming darkness, and the life signs become scarce. If you go deep enough, you will lose any trace of life, with a mountain of water above you and the chasm beneath you. But you can still go deeper. It will be so dark you won't know whether you have eyes opened or closed, and while you initially will take the sounds as blood in your ears pounding under pressure, the closer you get, the more you will recognize the distinct sound of drums propagating through the water. It will take many times over the distance from the surface to the last lifeform before you see anything, but eventually, you will see a glimmer. When you pursue it, you will arrive at a crystal palace. His minions will stand guard. With their soulless eyes in scarred, sick bodies, they will not pay attention to you.
As you enter the stronghold, you will discover a maze of stone and mirrors. And as you explore it, you will see reflections and movements at corners of your sight, and very soon, you will discover that you're not the only one in the castle. But, to your terror, you will notice that in the crumbling darkness you lost your way, and even if you escaped the place, you are too far beneath the surface to hope to see it again.