I\'m not sure if PS really warrants an entry in an encyclopedia, but whatever.
Anyway, AFAIK, Yliakum is the \"world\" of the Yliaki (people of Yliakum). It is the inside of the stalactite (but also refers to the stalactite itself OOC-ly). Thus, while it is country-sized, it is the \"world\" for most Yliaki. However, the game world is different from it because it consists of multiple places, including the cave where the stalactite hangs in, the stone labyrinths, the surface of the planet the cave with the stalactite is on, and even other planets or comparable places (called \"planes\").
Hydlaa and Ojaveda are cities in the land / world of Yliakum.
As for the waste conduits: these are not the sewers. The sewers are very much like the RL ones. They probably end in the waste conduits, but they are not them. The waste conduits are located only at the outer walls, because otherwise they would reach into the levels below, obviously. Thus, they are basically in the walls of the stalactite. This, of course, brings up a set of problems:
1) How do things from the center of the levels get there?
2) If they are in the walls, they are not infinite, thus they will either have to go into the cave (dumping all waste on Pradesha) or slowly fill up
If they were inside the upper levels only, which would permit them to be reasonably infinite, then the problem of 1) is even more severe as the lower levels would have to have some way to get their waste up.
These are not unsolvable, but since nothing is mentioned in the background of PS yet, it can only be speculation.
