Play Chapter 6 "The Fall" and reach the 1971-era test facility. Cave will begin talking about testing on hobos. Below one of the test spheres and a giant Aperture logo is a room marked "CONTROL ROOM" in yellow letters. Use a portal to get high above the Control Room and enter the small office. There is a cabinet with a door behind it in the corner of this office. Enter that door and follow the hidden hallway to a dock. At the end of the walkway near the docks is a orange life preserver marked "BOREALIS". Examine it to earn the achievement. Note: This is a reference to the Aperture ship "Borealis" that is seen at the end of Half Life 2: Episode 2.