The Free On-line Dictionary of Computing (19 January 2023):

salt mines

   Dense quarters housing large numbers of programmers working
   long hours on grungy projects, with some hope of seeing the
   end of the tunnel in N years.  Noted for their absence of
   sunshine.  Compare playpen, sandbox.

   [Jargon File]


The Jargon File (version 4.4.7, 29 Dec 2003):

salt mines
 n.

    Dense quarters housing large numbers of programmers working long hours on
    grungy projects, with some hope of seeing the end of the tunnel in N years.
    Noted for their absence of sunshine. Compare playpen, sandbox.