They came from the stars
They Came From the Stars is an example setting for Cybergempunks.
For centuries, man has studied the stars. Wondering what their purpose was. Speculating if there was a world beyond them. People sailed to the edge of the world to investigate. Those few who returned alive spoke tales of each star being a small circle of light surrounded by stones inhabited by strange and wondrous beings. More advanced telescopes allowed people to verify that the sky was indeed a perfect sphere, with the bright disc of the sun on one pole and the darker disc of the moon on the other. Ascent of the Great Mountain allowed cartographers to map the roughly triangular shape of the world, and see the water vanishing into the sky on all sides.
Not soon after space explorers flew to the top of the sky and determined that there were indeed creatures living there, a submarine expedition to the bottom of the ocean revealed that eventually gravity flipped directions and there was an entire desolate world beneath our own. Travelers to this other side discovered gems of a fourth color, Black, and experimentation eventually revealed that using them allowed one to project an avatar into the sky to communicate with its people.
Geometry of the World
The planet is a roughly triangular connection of continents, surrounded by a large ocean. At the very center of the region, there is a tremendous mountain, from which large quantities of air and water constantly emanate. Gravity pushes downwards, perpendicular to the plane of the oceans. The other side of the planet is approximately the same, although life does not appear to have evolved there.
The sky is a perfect sphere which displays the contents of the astral plane. Objects touching this sphere are pushed back gently, but touching with any significant force annihilates the touching part of the object completely. Creatures on the astral plane cannot observe the planet from their vantage point.