mines イディオム
back to the salt mines
back to work, humorous expression to express going back to unpleasant work Well, lunch is over so let
back to the salt mines|back|mine|mines|salt|salt m
informal Back to the job; back to work; back to work that is as hard or as unpleasant as working in a salt mine would be.

An overworked phrase, used humorously.
The lunch hour is over, boys. Back to the salt mines! "Vacation is over," said Billy. "Back to the salt mines."