Underland

Underland was the name for all the land under the surface of the world of Narnia. The top was called the Marches, the Deep Realm was much further underground, and Bism was another six thousand feet below them. Underland was inhabited by the Earthmen who were from Bism, a land of molten rocks; they called the Marches and the Deep Realm the "Shallow Lands" due to them being a lot higher than their natural home. All outlets from Underland to the Overworld lay across the Sunless Sea (except for the tunnel dug by the Earthmen under the spell of the Lady of the Green Kirtle). After the Lady of the Green Kirtle was slain and the Earthmen returned to Bism, almost all of the shallow Lands had flooded. However, since the other tunnel was conected to Narnia, south of the northern Marshlands, the water was lowered a little, and the Sunless Sea became a part of the Narnian Empire.