We have discussed the backside of the planet before, debating whether Athas is just a flat earth disc, tidally locked, or a globe that just hasn’t had the backside fleshed out… I believe I made a post about that a few months ago…
Silt and sand actually trap large amounts of water, it is very likely that below the Silt Sea lie vast amounts of water at the very bottom. As I said in another thread, they make sand dams in Africa that trap water between the silt and sand and the soil layer keeps the water from evaporating.
I had a whole chapter of a campaign in the DS “Underdark”, with the players fighting kenkus and yuan-tis. They also stumbled on a ruined underground complex with undead and shadows (mindless shadows of various types and an intelligent shadow boss - think of a shadow meorty).
Hej-kins, ankhegs, spiders, and cave fishers, already mentioned, are also good underground denizens, as mentioned above in this topic.