Overview
In order to get this achievement/easter egg, play through Chapter 6: The Fall until you get to the old testing facility. By the time Cave will start talking about testing on hobos and with that said you will know you are in the right area. Below one of the test spheres and a giant Aperture logo in its cooper Black Font is a room marked by the name CONTROL ROOM in yellow color. You have to use a portal to get above the Control Room and enter a small office. In the corner of this office you will find a cabinet with a door behind it (its really hard to miss). Enter this door to find a secret hallway that leads to a dock. At the end of the walkway by the docks, you will find an orange life preserver marked “BOREALIS.” Examine this to get the Achievement / Trophy.
The Reference
This is a reference to the Aperture ship Borealis seen at the end of Half Life 2: Episode 2.