A Salt Mine Looks Like From The Inside

Before engines and earth-moving equipment were invented, mining salt was one of the most expensive and dangerous things to do.

While salt is now plentiful, until the Industrial Revolution it was difficult to come by, and salt mining was often done by slave or prison labor and life expectancy among those sentenced was low.

In Roman times, salt on the table was a mark of wealth; so valuable was this resource that soldier's pay was originally in salt. This is where the word 'salary' comes from. 

Source Reddit