Borealis

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.

Portal 2
In Chapter 6: The Fall, the Borealis' drydock can be found in Test Shaft 09,. 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.