go down vs plunge

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

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

As verbs, plunge is a hyponym of go down; that is, plunge is a word with a more specific, narrower meaning than go down:

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

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