The Borealis is a scientific research vessel owned by Aperture Science. It first appeared in Half-Life 2: Episode 2, but a reference to it can be found in Portal 2 as an easter egg hidden in Chapter 6. The easter egg is located in close proximity to the office in which Chell finds GLaDOS, embedded in a potato, being pecked by a bird.
In Chapter 6: The Fall, the Borealis' drydock can be found in Test Shaft 09, during the 1970s section. It is hidden behind a door in one of the offices and, while the drydock is empty, life preservers with the word 'Borealis' printed on them can still be seen.
Trivia
Blueprints of the Borealis can be seen in Portal 2Co-op, and appear to have been signed by GLaDOS herself.
The Borealis was originally to be featured in Half-Life 2, but was removed when its story arc was cut from the game.
