The Land of Bism appears in The Silver Chair. It is a magical land made of molten rock shaped into terrestrial life, where gems grow on trees and there are streams and waterfalls of liquified ores. I’ve heard it said that the name came from the word abysmal, a descriptor of great depth; but, frankly, Bism is a happy place, not the dreary hellhole abysmal implies. Therefore, I propose a simpler origin: a contraction of “Burning Chasm” with a vowel change.
Bism has other connotations. There is the element Bismuth, which when oxidized develops a rainbow iridescence, which could apply to a land looking like “a tropical sun through a stained-glass window.” There is also the German battleship Bismarck, sunk in a famous naval battle in WWII at a depth of 15,719′, which sounds pretty deep to me.
Were there other Bisms, or flaming geographic divisions within Bism? Perhaps. I came up with this list of names based off words for vulcanism, depth, and heat.