tunnel vs underpass

tunnel and underpass both are nouns.

Word	Noun	Adjective	Verb	Adverb
tunnel	Yes	No	Yes	No
underpass	Yes	No	No	No

As nouns, underpass is a hyponym of tunnel; that is, underpass is a word with a more specific, narrower meaning than tunnel:

tunnel: a passageway through or under something, usually underground (especially one for trains or cars)
underpass: an underground tunnel or passage enabling pedestrians to cross a road or railway

Other hyponyms of tunnel include catacomb, railroad tunnel, subway.

tunnel (noun)	underpass (noun)
a passageway through or under something, usually underground (especially one for trains or cars)	an underground tunnel or passage enabling pedestrians to cross a road or railway
a hole made by an animal, usually for shelter

tunnel (verb)	underpass (verb)
force a way through
move through by or as by digging