A typical island in Xen.




Small; exact measurements unknown.

Xen, or the Borderworld, is a separate overlapping dimension notably discovered by Black Mesa scientists, who regularly traveled there via teleport to collect samples and study the myriad species of flora and fauna within. It is the presumed home of the Vortigaunt race, and a place where many of the alien species that migrated to Earth during the beginning of Xeno War I originated from. It is partially merged with Byaha, and completely surrounds the universe.

Rather than being a planet or an artificial structure such as a space station, most of Xen appears to consist of asteroid islands floating within a nebular void. Xen is notable for having greatly reduced gravity compared to Earth. However, given that Xen is comprised of low mass asteroids, it is surprising that there is any appreciable gravitational pull at all, let alone a pull strong enough to retain the breathable atmosphere encountered there.

Another feature is that gravity is always directed downwards, rather than towards the asteroid, so that one can actually fall off an asteroid into the void below. While it is possible that the "islands" are small chunks orbiting a larger and unseen planetoid, these unusual physics strongly suggest that Xen occupies an alternate dimension where conventional physics may not fully apply.

The Xen asteroids are home to a wide variety of flora and fauna, forming a complex ecosystem. While mostly seen as lifeless, the presence of water (reddish-green in color) indicates that the dimension is somewhat capable of supporting life. Some species, such as Barnacles, Bullsquid, Headcrabs, and Houndeyes appear to occupy niches similar to various carnivorous terrestrial animals, albeit with unusual anatomical or physiological features. However, even these creatures are dwarfed by the towering six meter high, four-legged invertebrate known as the Gonarch, which appears to be the final stage of a Headcrab's life cycle.

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