Xen (also labelled The Borderworld) is a plane of existence connecting two or more dimensions. It was discovered by Black Mesa research scientists, who regularly traveled there via teleportation to collect samples and study the myriad species of flora and fauna within, despite the extremely high risk of doing so.
Rather than being a planet or an artificial structure such as a space station, all of the locations of Xen are on the surfaces of (or within) asteroid-like "islands" floating within a nebular void.
Xen is notable for having greatly reduced gravity compared to Earth's; however, given that Xen is composed 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. It is possible that, given that Xen resides in another dimension or universe, the entire void contains a breathable atmosphere.
Another feature is that gravity is always directed downwards, rather than towards the asteroids, so that one can actually fall off an asteroid into the void below.
Light behaves oddly in Xen. There are multiple sources for light in the borderworld, either through glowing clouds of gas or (in some cases) what seems to be a star or some other stellar body.
The Xen asteroids are home to a wide variety of flora and fauna, forming a complex ecosystem. All of these species originated elsewhere and were forced to flee there to escape attack from their enemies. While mostly seen as lifeless, the presence of water (reddish-green in color) indicates that the dimension is capable of supporting life.
The intelligent species of Xen, such as the Alien Controllers and Vortigaunts, form part of an advanced alien civilization, with species like the Alien Grunts and Gargantua, composing their military. As the player progresses through Half-Life, it becomes clear that this civilization is controlled by a central intelligence, the Nihilanth. Although differing in many ways, the Controllers, Vortigaunts, Grunts, and the Nihilanth share a common morphology that includes a vestigial third limb in the middle of the thorax, suggesting that they are all closely related.
A few creatures that appear in Xen are difficult to categorize as either fauna or flora due to their unusual appearance. Stationary bio-luminescent stalks appear in large numbers throughout Xen and will retract when approached. The much larger and extremely aggressive (but rarely encountered) Tentacles pose a major hazard to those who wander near their pits. There are also bizarre tree-like structures dotted around the landscape, which may be juvenile or even a separate species of tentacle. The "trees" sway gently and appear harmless when observed from a distance, but stab viciously at any creature that enters their immediate vicinity. There are also strange trampoline-like plants seen in Black Mesa and Xen that release a blast of air when stepped on. A similar kind was seen on Xen which opens and closes, releasing a gust of air like a vent.
Colonization and civilizationEdit
After the Combine took over the Xen residents' former world, the fleeing aliens retreated to Xen via teleportation and colonized Xen with their buildings and technology (also bringing with them invasive wild-life and pests). The buildings for the Vortigaunts were primitive and lacked luxury or necessary needs of survival (besides the healing pool in the back) due to their social class as slaves. The Vortigaunt's buildings look like small hills carved by an unknown tool.