What is the palace on the floor of the great sea Haloisi Velike in Tolkien's early drawing "I vene kemen"

Question

J.R.R. Tolkien's early (1916-1920) drawing I Vene Kemen is a depiction of Arda (not called that yet) in the form of a ship:

Christopher Tolkien included this drawing in The Book of Lost Tales, Part I, showing Tolkien's earliest version of his mythology.

On the floor of the Great Sea (which would later be called Belegaer) appears a palace with the label Ô next to it.

The words Haloisi Velike appear just above the surface. Tolkien Gateway translates Haloisi Velike as "Great Sea" and Ô as "Sea", which would argue against Ô being a label for the palace.

This is decades before Tolkien conceived of Númenor.

What is this palace? It could be the halls of Ossë or Ulmo, or some early idea of either one.

Answer 1

This is the dwelling of Ossë beneath the Great Sea

1. Ossë's dwelling is described as being beneath the Great Sea, just like this one appears on the map

   Haloisi Velike, which this palace appears under, is glossed by Christopher Tolkien as 'The Great Sea'.

   Haloisi Velike 'The Great Sea'.
   _{The Book of Lost Tales Part One, "Commentary on The Coming of the Valar and the Building of Valinor"}

   In "The Chaining of Melko", Ossë is said to have a "dwelling beneath the Great Sea."

   Three great fish luminous in the dark of the sunless days went ever with Ulmo, and the roof of Ossë's dwelling beneath the Great Sea shone with phosphorescent scales. Behold that was a time of great peace and quiet, and life struck deep roots into the new-made soils of Earth, and seeds were sown that waited only for the light to come, and it is known and praised as the age of "Melko's Chains".'
   _{The Book of Lost Tales Part One, "The Chaining of Melko"}

2. This cannot be Ulmo's halls, as Ulmo lives in the Outer Ocean (Vai), not the Great Sea, and Ulmo's halls are also explicitly labeled on the map, in the Outer Ocean

   Ulmo lives in the outer ocean, leaving the Great Sea to Ossë

   But Ulmo dwells in the outer ocean and controls the flowing of all waters and the courses of rivers, the replenishment of springs and the distilling of rains and dews throughout the world. ... Salmar there was with him, and Ossë and Ónen to whom he gave the control of the waves and lesser seas, and many another.
   _{The Book of Lost Tales Part One, "The Music of the Ainur"}

   Ulmo arose and went to the Outermost Seas that were set beyond the Outer Lands. He loved not high words nor concourse of folk, and in those deep waters moveless and empty he purposed to dwell, leaving the governance of the Great and lesser seas to Ossë and Ónen his vassals. Yet ever of his magic deep in his outermost sea-halls of Ulmonan he controlled the faint stirrings of the Shadowy Seas, and ruled the lakes and springs and rivers of the world.

   [Commentary by Christopher Tolkien:] It has been said in The Music of the Ainur (p. 58) that Ulmo dwells in the Outer Ocean and that he gave to Osse and Onen 'control of the waves and lesser seas' ... It is now seen that Ulmonan is the name of his halls in the Outer Ocean, and also that the 'lesser seas' controlled by Ossë and Ónen include the Great Sea.

   _{The Book of Lost Tales Part One, "The Coming of the Valar and the Building of Valinor"}

   Ulmo's halls, Ulmonan, are explicitly labeled on the map, where they appear on the left, in the hull of the ship, distinct from the structure that you are asking about. (Scroll down for close ups.)

   
   _{Tolkien: Maker of Middle-earth - "I Vene Kemen’ – ‘The Map of the Ship of the World’"[no.68]}

   
   _{The Book of Lost Tales Part One, "Commentary on The Coming of the Valar and the Building of Valinor"}

3. Christopher Tolkien has identified this structure as Ossë's dwelling in his commentary that was printed alongside this map in The Book of Lost Tales.

   In The Book of Lost Tales Part One, which until quite recently was the only place this was published, it was accompanied by a commentary from Christopher Tolkien, going through each term in the drawing.

   I give here a list of the names and words written on the drawing with, so far as possible, their meanings (but without any etymological detail)
   _{The Book of Lost Tales Part One, "Commentary on The Coming of the Valar and the Building of Valinor"}

   Next to Christopher's entries for Haloisi Velike and Ô he also discusses the nearby structure and says that it "must surely be the dwelling of Ossë".

   Haloisi Velike 'The Great Sea'.

   Ô 'The Sea'. (What is the structure at the sea-bottom shown below the name Ô? It must surely be the dwelling of Ossë beneath the Great Sea that is referred to in the next tale (p. 106).)
   _{The Book of Lost Tales Part One, "Commentary on The Coming of the Valar and the Building of Valinor"}

   Christopher's reference to "the next tale (p. 106)", is to the final paragraph of "The Chaining of Melko", which has been given above at the start of this answer.

Bonus: Close up pictures of both dwellings, in both Tolkien's original, and in Christopher's redrawn versions.