cave in vs implode

cave in is a noun but implode is not a noun.

Word	Noun	Adjective	Verb	Adverb
cave in	Yes	No	Yes	No
implode	No	No	Yes	No

As verbs, implode is a hyponym of cave in; that is, implode is a word with a more specific, narrower meaning than cave in:

Other hyponyms of cave in include go off, buckle, crumple, flop, break, sink, slide down, slump.

cave in (noun)	implode (noun)
the sudden collapse of something into a hollow beneath it

cave in (verb)	implode (verb)
break down, literally or metaphorically	burst inward