Do you know that Roman soldiers are paid in salt?  Hence the word "salary" comes from Latin, word for salt, "salarium".  This phrase probably came from Tzarist and Communist Russia sending prisoners to work in salt mines in the 19th and 20th centuries.

Idiom # 3

" Back to the salt mines "

A humorous way of talking about returning to work with reluctance.

" After 2 weeks off, now is the time to go " back to the salt mines!"