So Half-Life is based on a true story then.
Half-Life series features a number of parallel universes from which both hostile and friendly alien species originate. One inter-dimensional alien race, the Combine conquers Earth and attempts to harvest and enslave humanity. Only in Half-Life and its expansion packs, however, does the player ever visit one of these parallel dimensions; the so-called 'border world', Xen
Xen is an alternate-dimension and is the adopted home of the Vortignauts. A collection of asteroids hanging over a nebula, Xen is briefly featured in Half-Life and its first two expansions, Opposing Force and Blue Shift. 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. The player encounters multiple types of fauna such as Headcrabs in Xen, 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 in Xen is significantly lower than on Earth. Xen forms the setting for the closing parts of Half-Life, where Gordon Freeman travels to Xen to kill the Nihilanth and seal the spatial fracture to Black Mesa. The player briefly visits Xen in both Opposing Force and Blue Shift as well; in the former, Adrian Shephard is forced to travel to Xen to escape an otherwise enclosed area, while in the latter, Barney Calhoun goes to Xen to align some equipment to allow Black Mesa to be evacuated using the teleporters. After the death of the Nihilanth in Half-Life, the fracture is destabilized further, causing large amounts of Xen's wildlife to be teleported to locations across Earth.