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.