treadmill vs salt mine

treadmill and salt mine both are nouns.

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

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

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