2021-03-03, 08:52 AM (ISO 8601)
Re: What would a literal "under-world" look like?
Disc-world shape, floating in the void is orbited by the sun and moon(s). The sun is smaller and nearer, so its orbit is daily. It can eclipse the moon, but because you can't see the moon when the sun is up, only astronomers on the edges of the world, (which appear as a massive mountain range to those who go there,) can observe a solar eclipse of the moon, though lunar eclipses are more frequent.
The moon is also a disc, and its top and bottom align with that of the world, so that it waxes and wanes as it travels across the night sky.
The atmosphere is a globe, thinner at the world's rim. When water falls from the world it breaks down to a mist which is heated by the close passage of the sun, forcing it to flow as steam back to the top where it falls as rain. The orbit of the sun is nearer in the summer and more distant in the winter causing the weather to be more regular in summer and mose violent in winter.
Nightly dews and fogs are the common source of water on the underside as steam which did not make it around the edge is drawn into the cooling center of the disc. Plants anchor to the rocks and dangle, capturing moisture in various ways.
Underside is the domain of flying things, but terrestrial creatures, especially brachiators, thrive in the dense jungles which form around the seeps and waterfalls. These 3D mazes are separated by open expanses not unlike moonscapes. The craters are where chunks of the world have broken off.
When a chunk breaks it forms an island with a mini-ecosystem not unlike that of the world. It slowly falls for decades or centuries, its top growing ever more lush as it is ground into dust by the elements, eventually to be caught in the steam clouds and deposited on the surface by the rain.