Not all space marines abide by the [insert name of order to kill all non-human species]. There are those who rebelled against the very idea. One of the factions is known as The Faithful.
The Faithful serve as the section of the rebellion who are tasked with rescue missions, saving as many as they can. They are more on the defensive side, harboring all they have rescued inside their invulnerable stronghold, The Helm. It's here that they keep them protected, especially in the field of preservation, having them breed. They have built a cavernous labyrinth under their fortress, having kilometers upon kilometers of intricate structures in a vast underground complex.