plunge vs come down

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

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

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

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

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

plunge (verb)	come down (verb)
begin with vigor	get sick
devote (oneself) fully to	be the essential element
cause to be immersed	criticize or reprimand harshly
immerse briefly into a liquid so as to wet, coat, or saturate	move downward and lower, but not necessarily all the way
thrust or throw into	fall from clouds
fall abruptly
drop steeply
dash violently or with great speed or impetuosity