You've delved deep into the earth, deeper than any before you, past twisting caves and voluminous caverns, beyond the boiling lakes of fire, and yet deeper still. The air is cooler now, and the unexpected scent of fresh air catches your nostrils. As you break through the last layer of rock and dirt, you're greeted by an open space. No, the open space. The vast expanse of the sky stretches out below in a dizzying drop into nothing. The bright orb of the sun briefly blinds you, indicating that it may be night on the surface above. After all this time, you've finally found what you could only have speculated on before: the world's underside!

I was thinking about this and initially thought that maybe the underside would just duplicate the surface, but upside down, but quickly realized that that wouldn't work for a number of reasons. Now, there are a couple different types of "underworlds". Is it a hollow-Earth or the underside of a flat world? If the former, is it a giant cavern or is there a sun-analog to give light? Is gravity reversed or does everything hang down from the earth above? For now, I'll just assume a flat world without reversed gravity (as reversing gravity would pretty much make the underside the same as the surface).

The first thing that occurred to me is that most animals probably either fly or are really good at climbing and jumping.

However, if we want to have life in the first place, rather than a barren wasteland, what we really need is water. Because the ground is upside down, there can't be any lakes or rivers. You can have streams that flow out of the underground, but it will just fall into the sky. I can imagine having a thriving ecosystem gathered around such points, likely with plants designed to catch as much of the falling water as possible, but these ecosystems would be fairly small. We also can't have rain, unless we contrive a mechanism by which the rain falls up instead of down (strong wind, perhaps?). This also begs the question of what weather would look like in such a place; I imagine rain falls down but lightning goes up.

What I suspect to be the mostly plausible method for watering this underworld is dew. Basically, mist/clouds form close to the "ground" and condense into water. I can see having plants that try to maximize surface area so as to condense the most water, or they just soak up the water and swell up really big (like succulents). Trees might have branches that bend back toward the underside, with leaves shaped like bowls to catch and hold as much water as possible. Perhaps vine-like plants hang down into the sky to try and capture any moisture from the air. I wonder if the clouds would be a lot closer to the underside than they are to the surface; if so, then it might be possible for such a vine to reach the cloud layer and siphon water out of the clouds.

Floating islands, if they exist, could also change things a lot. They would give a place for things like lakes and other features that could only exist on the surface to form. These floating islands might catch the water flowing out of underground streams, and the vine-things might hang down into the lakes and suck the water back up to the underside. Animals could also drink from these lakes, and might even make homes on the floating islands.

That's about as far as I've thought this out. Lots of hanging plants and stuff, streams flowing out of the underground that are hotspots for life, and other weird stuff that is designed to make life on the underside plausible, if fantastic. Figuring out where the water comes from seems like the biggest first step, which will then lead to emergent world building where plants and animals will gather near the water and various systems and subsystems will build themselves as a natural consequence to these things.