Once, long ago, the far flung regions of the Metal earth were connected by a network of teleportation pools. Traders and travelers journeyed via the pool system, utilizing submersible barges to pass through what was known then as The Between Sea. The trip was so short, though, that in most cases, teams of swimmers, usually comprised of Reptarch Longfin (now thought to be extinct), could tow all but the largest items from place to place. Able bodied humanoids of every race had no trouble making the trip. Like so much else, this golden time is all but lost to memory- today, most people don't even know the pools exist.
During the current time period The Between Sea is tainted and unsafe for travel. Huge undead angler fish, known as ghostfins prowl the waters, as well as a host of other evil and undead creatures. Distances seem longer as well. What once took but a few moments now requires hours- or longer. The pools are still usable, but the passage is very dangerous, and few who enter the water came out.
Pools range from about the size of bathtub to slightly larger than an Olympic swimming pool. Many cities have them, but few residents- even those in charge, know they exist. Evil creatures such as the Changers and the Slavers of Gear make use of them to further their vile and twisted schemes.
Aside from the large-scale pool network many large sites featured closed networks of pools. These may or not share the taint of the open Between Sea.