I don't know. I've never been there. Nor has anyone I know. I'm guessing you'd fall off that side though.
So, we can hypothesise. And your hypothesis is every bit as good as mine.
We could start out conservatively, imagining it would be rock ... maybe some kind of vegetation that grows downwards on its surface? Or you can just let your imagination run away with you. Terry Pratchet imagined a turtle on the backs of 3 elephants. H G Wells decided on Morlocks. The Ancient Greeks went with an underworld and the Jews liked the idea of hell. I'm going to go with a lush paradise, filled with nymphomaniac Amazons who are 7 feet tall with huge bobs. They enjoy cooking, beach volley ball, brewing their own beer and giving deep tissue massages ... when they aren't having sex of course. Oh, and they love male visitors from the top side of the disk. You can see why no one has ever returned ... I mean why would you?