salt mine vs treadmill

salt mine and treadmill both are nouns.

Word	Noun	Adjective	Verb	Adverb
salt mine	Yes	No	No	No
treadmill	Yes	No	No	No

As nouns, salt mine and treadmill are synonyms defined as:

salt mine (noun)	treadmill (noun)
a job involving drudgery and confinement	a job involving drudgery and confinement
a mine where salt is dug	a mill that is powered by men or animals walking on a circular belt or climbing steps
	an exercise device consisting of an endless belt on which a person can walk or jog without changing place