tumba noun "deep valley" (Letters:308; SA:tum and TUB gives tumbo "valley, deep valley"); apparently an extended form *tumbalë in tumbalemorna "deepvalleyblack" or (according to SA:tum) "black deep valley", also tumbaletaurëa "deepvalleyforested"; see Taurelilómëa-tumbalemorna...
Quenya
tumba
adjective. deep valley, [ᴹQ.] deep, lowlying; [Q.] deep valley
tumba
deep valley
tumbo
(deep) valley
tumbo (stem *tumbu-, given the primitive form ¤tumbu) noun "(deep) valley", under or among hills (TUB, SA:tum), "depth" (PE17:81). - In early "Qenya", the gloss was "dark vale" (LT1:269). See tumba.
tumba-
verb. to cast down (into depths); *to dive; (orig.) to go or cause to go deep, endeepen
lún
deep
lún adj.??? a word of obscure meaning, perhaps "deep" as used of water (VT48:28)
núra
deep
núra adj. "deep" (NŪ)
The adjective ᴹQ. tumba “deep, lowlying” appeared in rough (and ultimately rejected) notes on irregular verbs from the Quenya Verbal System of the late 1940s as a derivative of ᴹ√TUB “fall low, go down” (PE22/127). In a 1961 letter to Rhona Beare tumba was glossed “deep valley” as an element in the Entish phrase Q. Taurelilómëa-tumbalemorna Tumbaletaurëa Lómëanor “Forestmanyshadowed-deepvalleyblack Deepvalleyforested Gloomyland” (Let/308; LotR/467), but I think this is only an approximate translation, and the word is better understood as adjectival in sense: “✱like a deep valley”. As further evidence of this, in notes from the late 1960s the form tumba was changed to a more typical noun form Q. tumbo in the name Q. i Tumbo Tarmacorto “the Vale of the High Mountain Circle” (NM/351).
Neo-Quenya: For purposes of Neo-Quenya, I’d treat this word as an adjective only, and use Q. tumbo for the noun.