The Mirror Pool was an enchanted body of water capable of making intellectually simplified copies of ponies. It was originally used by Equestrian forces to clone an indefinite number of soldiers until it was destroyed by Zebra infiltrators in the early stages of the war.
The destruction of the pool caused all of the clones to vaporize, allowing the zebras to overwhelm and destroy Big Macintosh's division.
It is mentioned only in records and flashbacks.
A major sub-plot of Source: Dirty Hearts regards the Department of Progress initiative to use the pool for military purposes. Some water from the pool, stored in a facility north of Canterlot, avoided destruction.
Records show that the more copies made of a single pony, the more simplistic each copy was, and the more each showed a smaller number of amplified character traits (i.e.- an already aggressive pony would produce increasingly aggressive copies, with later copies being little more than enraged psychopaths).
Backbone and Skully have to salvage a maneframe board from the facility where these experiments took place. They fight through a labyrinth now overrun by near-mindless copies of six prior staff members, all of whom have lapsed, in absence of any other food source, into cannibalism.