On the skin of the world the races of light live and love, fight and die beneath the sun, moon, and stars. But there is another world beneath their feet. If you could sink down through the earth you would first find the land of earthworms and moles, full of animal burrows and networks of roots digging deep. This is the thinnest tissue covering the greater depths. Beneath it lies the realm of stone, where dwarven tunnels twist in search of gold, mithril, and gems. But deeper still there is another realm. A strange and dangerous place. When the dwarven tunnels encounter it, the dwarves block them up, no matter how rich the seam they followed there.
They call that place the Caverns of Madness.
The sunlight never reaches there, but it is not always dark. Fungi grow in great, luminous forests, lighting many of the deeper caverns. Strange insects twinkle like stars where they cling to the rock. And there are places with lights that are stranger yet. Caverns lit with a dim, sourceless glow that seems to come from the air itself. Caverns where apparent sunlight streams down as though the sky lay overhead, though miles of stone may lie between them and the world above. And other, less easily described places, where unspeakable colors cast indescribable lights and shadows on the cavern walls.
The inhabitants of this realm are many and varied. Most were born there, perhaps created by the bizarre ancient gods who are said to have made the caverns themselves. Some have crawled down from the world above, but those do not remain unchanged, and most such would not be recognized by their light-dwelling kin.
Some of the tunnels and caverns seem purely natural. Others have been built by sentient hands, or by things perhaps less sentient, colonies of mindless burrowers and builders like and yet profoundly unlike the ants, termites, and bees of the sunlight world. Those inhabiting any given area are not necessarily those who built it. The only constant in the Caverns of Madness is change.
Great civilizations have risen there, and fallen too. In some places vast, bustling cities fill massive caverns, or crawl through tunneled mazes like giant ant's nests. In other places abandoned cities lie fallen to dusty ruin, the debased remnants of their former builders living in squalor amid the fading glory of their ancestors. Hunting tribes of bizarre creatures roam from cavern to cavern, and sessile sages squat, glued to the stone itself. Anything and everything might live there.
The magic there is as twisted as anything else. In the surface world magic lies like a vast shallow sea, everywhere much the same. But in the Caverns of Madness magic itself has gone mad. Large areas may abide by the same rules, but the same spell, cast in two spots mere yards apart, might have two wildly differing results. What causes a fireball on the surface might cause a devastating explosion in the depths, or harmless foxfire, or a shower of fireflies, or a flood of gelatin, or any number of peculiar results. Any given area has its own rules, but the rules differ wildly from place to place, so no traveling mage can depend on his magic.
These, then, are the Caverns of Madness, a thousand tiny worlds, each with its own inhabitants, its own rules, and its own dangers. Explore them if you dare. ( Yeah, I'm doing it. Collaborative world is GO. )
*crosses fingers* I really hope something fun and creative comes of this. I really, really do.