Xen is an alternate dimension discovered by Black Mesa scientists prior to the original Half-Life. Black Mesa sent expedition teams into Xen to collect alien samples, and is the source of the crystal that triggers the resonance cascade early on in the game.
Xen consists mostly of organic, spongelike platforms floating in a nebulous void. Various energy spires and other organic growths are commonplace. Xen occasionally features larger cave systems underground. The gravity on Xen is lower than on Earth.
Xen was often criticized for its frustrating "jumping puzzles". Therefore, it is best to relegate moving platforms to an atmospheric role rather than a gameplay mechanic.
Xen's lighting consists mainly of glowing crystals and bio-luminescent tubes. Odd fungi and pillars and can be seen spread around in areas.
No humans should ever be seen in a Xen level. Occasional dead expedition members can create a disturbing and desolate atmosphere, however.