In the movie Easy A, the character Todd said "See you at salt mines" to Olive when they met at Melody's party. What does salt mines mean? Does it really mean "a mine for salt"?
The American English expression "salt mine" is most often used to refer to tedious work. This use makes sense given the literal origin of the phrase. However, in actual usage, most people who use idioms or word pictures to express feelings do so with a bit of irony and are not necessarily saying that their work is really dangerous like salt mines nor that it is really all that tedious. Use of a phrase like "back to my salt mine" or "it is my salt mine" is helpful in acknowledging the feeling of much work to do while at the same time recognizing that the work really is not nearly so tedious as a real salt mine. So, in effect, it can be a motivating and upbeat way to keep your work in perspective, while still acknowledging that work is work.
I hope this explanation helps. I'm a word picture person myself and have realized that not everyone speaks in word pictures. I actually used the salt mine expression yesterday and ended up having to explain it.
Salt mines are, of course, just mines for salt, but in classical times, they were staffed by slaves, typically POWs, and the work was so arduous and the environment so dangerous, that being sent to the salt mines was considered a death sentence.
To describe any job as a "salt mine" is a hyperbolic way of saying the work is unpleasant, arduous, or repetitive.
edit reading John's reference, I realized I was confusing salt mines with classical tin mines (also very unpleasant). Salt mines are more recent and their staff, not necessarily POWs.
Notwithstanding the foregoing, it should also be noted that salt is significant in this phrase because of its value in ancient times. In ancient history, salt was prized over gold (http://www.smithsonianeducation.org/educators/lesson_plans/currency/essay2.html). Salt is required to sustain life and is still used to preserve food. Salt mines were more ubiquitous than gold mines.
While working in a gold mine as a slave may be every bit as arduous as working in a salt mine as a slave, the difference is that working in a salt mine sounds a lot less glamorous, which adds to the intent of the expression, which is to say, that one is going back to work at something unpleasant, repetitive, and ... completely dull and unglamorous.
Moreover, its also fitting because the English word for salary comes from the latin word 'salario' which means salt (http://en.wiktionary.org/wiki/salario)
Don't discount the simplest explanation: that there may actually be a place called Salt Mines, which should have been capitalized.
For instance, there is a place called Salt Mines here, in Northern Ireland. It's not a salt mine any more: it's a park, which has cycling routes / jumps, with a golf course and so on nearby. In other words, it's a place where people hang out; a place where two people might very well see each other later.
This is all assuming that you didn't leave out the word "the", of course. If it's "see you at the salt mines", then yes, they're probably calling the school a salt mine, as a half-joke.
The origin of the phrase is a real mine for salt -- specifically a mine located in Usolye (which is an archaic Russian word that just means "salt producer" or something similar), whihc is in Siberia. This particular mine was owned by the Russian ruling family (before it was overthrown in the October Revolution).
As far as why the phrase is used: because the mine was used primarily as a prison, especially for political prisoners. Conditions were reputedly exceptionally brutal, including housing that was completely inadequate to protect from the Siberian weather, and excruciating punishments, up to and including being flogged to death for failing to work hard enough (where "hard enough" was apparently defined quite arbitrarily).
To summarize: forced hard labor, brutal conditions, and capricious punishment.