"Our peers at Aperture Science were at work on a project of some promise, but in their rush to beat Black Mesa for funding, they must have compromised ordinary standards of risk. We heard their research vessel had simply disappeared. Vanished with all hands... even part of the dry dock! "
The Borealis is an Aperture Science research vessel introduced in Half-Life 2: Episode Two. According to Isaac Kleiner, Aperture was working on a promising project, but in their rush to beat Black Mesa for funding, they neglected ordinary safety rules and the ship simply disappeared with parts of its drydock, which earned it a legendary stature. It is assumed that the Borealis contained an immensely powerful and dangerous secret, most likely involving portals and teleportation on a larger scale than that of a handheld portal device, which is likely the reason the Combine are searching for it as they could possibly connect back to Combine Overworld and flood the planet with their large and powerful armies. The Combine also have yet to perfect localized teleportation, as the Black Mesa scientists and Aperture Science have, giving them ample reason to investigate the ship.