plunge vs go down

plunge is a noun but go down is not a noun.

Word	Noun	Adjective	Verb	Adverb
plunge	Yes	No	Yes	No
go down	No	No	Yes	No

As verbs, go down is a hypernym of plunge; that is, go down is a word with a broader meaning than plunge:

Other hypernyms of plunge include come down, descend, fall.

plunge (noun)	go down (noun)
a brief swim in water
a steep and rapid fall

plunge (verb)	go down (verb)
begin with vigor	grow smaller
devote (oneself) fully to	stop operating
cause to be immersed	be defeated
immerse briefly into a liquid so as to wet, coat, or saturate	be ingested
thrust or throw into	go under
fall abruptly	disappear beyond the horizon
drop steeply	move downward and lower, but not necessarily all the way
dash violently or with great speed or impetuosity	be recorded or remembered