Portal 2 Easter Egg - Borealis Drydock

At the end of chapter 6 (in the area where you get POTaTOS), go to the bottom floor of the building POTATOS is in. In the back left corner there is a door half hidden behind a book case. Go through the door, continue to the end of the hallway and go through the blown-open door frame to your right. You will be at a dock with several life-savers with the word "borealis" on them, which is a reference to the missing experiment in Half Life 2: Episode 2.