The Northwest Passage was the term given to a corridor-like region of space containing large numbers of quantum singularities, devoid of Borg activity despite extending through the entire span of Borg territory. The nickname was created by the crew of the USS Voyager, who intended to use it to cross Borg space without having to engage the Borg.
However, it was soon discovered that the region was devoid of Borg activity for a reason: the quantum singularities were being used by Species 8472, a race more powerful than the Borg, to invade the galaxy from their realm of fluidic space. This species, capable of completely annihilating the Borg, were bent on doing exactly that, in retaliation for the Borg's invasion of their realm and attempt to assimilate them. Moreover, they planned to expand their campaign to include all other life in the galaxy, after they were finished with the Borg. Captain Janeway therefore decided to avoid the corridor entirely, and instead make an alliance with the Borg to defeat Species 8472. (VOY: "Scorpion")
Named after the Northwest Passage, a corridor through Canada linking the Pacific and Atlantic Oceans on Earth by passage near the Arctic circle.
It is uncertain what happened to the corridor after the Borg-Species 8472 War had concluded. As Species 8472 withdrew to their realm, it is possible the singularities became sealed, yet the corridor may still be detectable by the stresses and instablilites in the fabric of spacetime that have undoubtably remained.