Tolkien used several similar roots over his lifetime as the basis for Q. Utumno and S. Udûn, the underground stronghold of Melkor. The earliest of these was unglossed ᴱ√TUM(B)U in the Qenya Lexicon of the 1910s with derivatives ᴱQ. tumbo “dale, valley” and ᴱQ. tumna “deep, profound, dark or hidden” (QL/95). It also had derivatives in the contemporaneous Gnomish Lexicon like G. tûm “valley” and G. tumla- “excavate, hollow out” (GL/71-72).
The root ᴹ√TUB appeared unglossed in The Etymologies of the 1930s with derivatives like ᴹQ. tumbo/N. tum “deep valley” and ᴹQ. tumna/N. tofn “lowlying, deep, low”, as well as ᴹQ. Utumno/N. Udūn (Ety/TUB; EtyAC/TUB). The root reappeared in a rejected page of roots from the Quenya Verbal System (QVS) written in 1948, where it had the verbal sense “to fall low[?], go down, below normal ground-level, esp. to go down (sink, dive) into water” (PE22/147). In this 1940s document the root had derivatives similar to those in The Etymologies, as well as a verbal derivative ᴹQ. tumba- “to cast down (into a pit[?])”. One indication that this verbal sense was not a new idea was the verb ᴱQ. tum- “dive” from Early Qenya Word-lists of the 1920s.
Tolkien’s continued use of both Q. tumbo and S. tum for “valley” indicate the ongoing validity of ᴹ√TUB, but in drafts of The Silmarillion from the 1950s Tolkien derived Q. Utumno from ✶Utupnŭ and the root √TUI, probably a malformed √TUP (MR/69); see the entry on √TUP for a discussion of that root.
An unglossed root in The Etymologies of the 1930s with derivatives like ᴹQ. tuo/N. tû “muscle, sinew; vigour, physical strength” and ᴹQ. tunga/N. tong “taut, tight (of strings, resonant)” (Ety/TUG). It was also the basis for the name N. Tuor “strength-vigour”, the only place where this name was given a clear etymology, though later Tolkien decided this name was actually from the language of the Edain (PM/348, 364 note #49).