Borealis

The Borealis was an Aperture Science research vessel. It was a boat docked in Cleveland. Probably due to Aperture Science's meddling in portals, the Borealis disappeared, along with its docks, never to be seen again. It soon became little more than legend. However, Judith Mossman discovered it years later but was captured by the Combine. Fearing that the Combine would gain the technology on the Borealis, Alyx Vance and Gordon Freeman went to rescue Mossman.

The Borealis will appear in Episode Three, though it is unknown what will happen to it: Eli Vance wanted to destroy it to keep the deadly technology away from everyone, but Isaac Kleiner thought the technology would be more useful than bad, and they often debated about it.

Cut material
The Borealis was originally a research ship stuck in the ice. It would have been crewed by a man named Odell (later Odessa Cubbage) and would have featured combat with Stalkers. This material, however, was cut from Half-Life 2. A mod named Missing Information features the Borealis, though.