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The Multiverse is an infinite set of universes, dimensions or planes of existence. It was mainly explored by Black Mesa, Aperture Science and The Combine.
- Earth Universe: where most of the events surrounding Gordon Freeman and Chell take place.
- Xen Borderworld: where Nihilanth was taking refuge from the Combine.
- Combine Overworld: where most of the Combine forces reside.
- Black Mesa: discovered Xen and repeatedly sent expeditions.
- Aperture Science: launched the Perpetual Testing Initiative as a mean to get new test chambers from other universes.
- The Combine: aims to submit every universe under its rule.