Earth

Earth is the third planet in the Sol planetary system and most notably inhabited by human beings, its dominant, native, and most intelligent species. Shortly after the humans' first development of teleportation technology, a spontaneous portal gateway was established with the Xen borderworld around the year 2000. This caused a major resonance cascade (at the location of the portal, during an "incident" at the Black Mesa Research Facility) that led to Earth's infestation of creatures from Xen (as well as habitation of a second intelligent race). The portal ultimately allowed members of the Combine Empire to invade and conquer the entire planet after a violent seven-hour conflict against the human race. The Combine eventually enslaved the human race and exploited Earth's natural resources, such as draining the oceans.

It is likely that the Combine's efforts to exploit Earth's resources will result in greatly increased global warming across the planet as well as Xen creatures, like Headcrabs and Antlions, suddenly inhabiting the planet might tip the ecological balance of Earth's wildlife resulting in more species going extinct and maybe even effecting human life.

A relatively small band of humans, known as the Resistance, united shortly after the Combine invasion to attempt to overcome the Combine's rule on Earth. The outcome of their efforts is unknown.

Black Mesa Research Facility
The Black Mesa Research Facility (or more commonly referred to as Black Mesa) was a research installation located in New Mexico in the United States, kept secret from the general public. It was where the Xen creatures first appeared on Earth because the resonance cascade occurred in a Black Mesa test chamber.

City 17
City 17 was one of many major Combine-installed cities on Earth. Exclusive to City 17 was the fact that Dr. Wallace Breen (administrator of Earth appointed by the Combine) established his base of administration there.

List of appearances

 * Half-Life
 * Half-Life: Uplink
 * Half-Life: Opposing Force
 * Half-Life: Blue Shift
 * Half-Life: Decay
 * Half-Life 2
 * Half-Life 2: Raising the Bar
 * Half-Life 2: Lost Coast
 * Half-Life 2: Episode One
 * Portal
 * Half-Life 2: Episode Two