A Quenya name of Númenor after its fall into the sea (S/281), it is simply the noun atalantë “collapse, downfall” used as a name (MC/223).
Conceptual Development: The name ᴹQ. Atalante appeared in the earliest tales of the fall of Númenor (LR/11, 25), usually in this form though at least once appearing as Atalantie (Let/347). Its resemblance to the name Atlantis was intentional (Let/347), created as part of the background for Tolkien’s (unfinished) time-travel story about Atlantis: “The Lost Road” (LR/36-104).
atalantë noun "downfall, overthrow, especially as name [Atalantë] of the [downfallen] land of Númenor" (DAT/DANT, TALÁT, Akallabêth, SD:247, 310; also LR:47, VT45:26). Variant atalantië "Downfall", said to be a normal noun-formation in Quenya (Letters:347, footnote). From the common noun atalantë "collapse, downfall" is derived the adj. atalantëa "ruinous, downfallen", pl. atalantië in Markirya (changed to sg. atalantëa this change does not make immediate sense, since the adjective undoubtedly modifies a plural noun, but Tolkien does not always let adjectives agree in number).