Mer City
The Mer City is an undersea city, home to the Mer people, where the A-B-Sea Serpent and Rattlesnake worked before their vacation.
First named in The Royal Book of Oz, ch. 10
They talked incessantly of the Mer City and told innumerable A-B-Sea stories to Scraps and the Tin Woodman and the children of the Emerald City.
Where this is
placed Beyond the desert worked out
read out of the chapters cited on this page, not stated in any one sentence of them
Royal Book of Oz ch2: the A-B-Sea Serpent describes coming 'from the sea depths up a strange river' to Oz — Mer City itself is undersea, outside Oz proper.