I can't find anything... I suspect that that term has been invented by someone who hasn't read the stories thoroughly. Perhaps he mistook "The Inner Sea" to be a reference to some kind of ocean inside Newhon, instead of simply one of the oceans on the surface, which it actually is.
The Caves of Ningauble are described as "a mazey lair", and contain gates to other worlds - when F & GM pass through one and end up in Tyre (Greece) about 300BC, their memories are altered to those of natives of the time & place they were then in. This story is
Adept's Gambit, which was actually the first F & GM story written, at a time when Fritz & Harry* had not yet created the World of Newhon - when the stories were collected in the "Swords" books, Fritx wrote an intro,
The Wrong Branch, which describes their jaunt into the Caves, and how the caves altered their memories as they went through the gate.
However, it is possible that the lands and oceans of Newhon are actually inside the planet, plastered to the inner surface of the sphere - making Newhon a "Hollow Earth"!
http://www.sf-encyclopedia.com/entry/hollow_earth
A bizarre alternative was proposed in the later nineteenth century by Cyrus Reed Teed (1839-1908), whose supposedly science-based creed of "Koreshianity" insists that the Earth is hollow and that we live on the inside, held to the ground not by Gravity but by centrifugal force as in a rotating Space Station. (The Sun is supposed to be artificial and internal, with apparent Stars being mere refractions of its light.) This cosmology is reflected in Fritz Leiber's Fafhrd and the Gray Mouser fantasies, whose characters believe – though the author does not explicitly endorse this belief – that their world of Nehwon is a bubble of air in an infinite sea, whose lands float on the inner surface.
A certain mind flayer may find this to be of interest:
Sea of Stars
Stories: Trapped in the Sea of Stars
The Sea of Stars, also known as the Great Equatorial Ocean, has a great eastward current, and is home to all the stars, the Moon, and the Sun, in the great champagne-bubble that is Nehwon.
http://www.scrollsoflankhmar.com/rpggui ... hyofnehwon
http://www.scrollsoflankhmar.com/rpgguide:rpgguide
* The characters were loosely modeled upon
Leiber himself and his friend
Harry Otto Fischer. Fischer initially created them in a letter to Leiber in September 1934, naming at the same time their home city of Lankhmar. In 1936, Leiber finished the first Fafhrd and Gray Mouser novella, Adept's Gambit, and began work on a second, The Tale of the Grain Ships. At the same time, Fischer was writing the beginning of The Lords of Quarmall. Adept's Gambit would not see publication until 1947, while The Lords of Quarmall would be finished by Leiber and published in 1964 and The Tale of the Grain Ships would become the prototype for "Scylla's Daughter" (1961) and, later, the novel The Swords of Lankhmar (1968).
Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam.
"I have a catapult. Give me your money or I will hurl a large rock at your head".
"Buffy, Blade... its up to you now." George Takei
The only time a Vampire should sparkle is right before they explode
Inner Earth: Is this Lankhmar's Underdark?
