Xen is an alternate dimension and is the home of the Vortigaunts. A collection of asteroids hanging over a nebula. It is often referred to as the "border world", as it is somehow involved in the teleportation process used by the Black Mesa Research Facility. Xen is the natural habitat to multiple types of fauna such as headcrabs, in addition to its sentient inhabitants. The asteroids are linked with their own teleporter system and a number of asteroids are shown to include underground factory-like areas, where Vortigaunts work to create or mature Xen's military forces. Gravity on Xen is substantially lower than on Earth. After the death of the Nihilanth in Half-Life, the fracture is destablised further, causing large amounts of Xen's wildlife to be teleported to locations across Earth.