Earth

Earth is the third planet in the Sol planetary system, most notably inhabited by humans. After the Black Mesa Incident, the planet becomes overrun by hostile alien creatures from the Xen borderworld, and later taken over completely by the Combine.

History
Shortly after the development of teleportation technology at Black Mesa, the Xen borderworld was discovered. Scientists began investigating Xen and experimenting on the creatures found there by travelling through portals to reach it. Experiments into mysterious crystals found on the borderworld led to a catastrophic event, dubbed the Black Mesa Incident, which tore open the gateways between the dimensions and allowed waves of aliens to spontaneously travel through and begin attacking the unprepared scientists.

This resonance cascade caught the attention of the Combine empire, who, after the detainment of Gordon Freeman and the destruction of the Black Mesa Research Facility began to send their own legions to Earth. A short but decisive battle ensued, later called the Seven Hour War, ultimately resulting in the enslavement of Earth's human population. The originally hostile Vortigaunts united with the humans and became part of the Resistance, an underground alliance who intended to overthrow the Combine and regain control of the planet. Bases were set up and over several years they gained much information on the Combine. The reappearance of Gordon Freeman led a sudden spur of movement and attacks from the Resistance, called The Uprising, allowing them to usurp control from the Combine forces in and surrounding City 17, destroying the Citadel Core and irradiating the entire surrounding area after escaping to bases in The Wasteland. City 17's destruction broke down the Combine's lines of communication, leaving them stranded within their settlements on Earth.

In an attempt to bring more forces through and thwart the Resistance, the Combine induced the Citadel's destruction, causing it to produce a Superportal. At White Forest, the Resistance were able to launch a rocket into space to take advantage of a satellite network to close the superportal before it matured and opened fully.

The Resistance has so far been successful in remaining covert and outside of the Combine's control, however as yet the outcome of their endeavors is unknown.

Black Mesa Research Facility
The Black Mesa Research Facility was an underground research installation located in the New Mexico desert. Notable for its scientific developments, such as portal technology and advanced weaponry, it was the epicenter of the Xen invasion. The research facility was later destroyed by the military in an attempt to control the alien infestation.

City 17
City 17 was one of many major Combine-installed cities on Earth, located in an unnamed European city and the base of administration of Dr. Wallace Breen, the puppet administrator of Earth appointed by the Combine.

Located in Eastern Europe, the city boasts characteristic architecture, though most of it has been either outfitted with Combine technology or destroyed altogether. The Combine Citadel, located at the center of the city, was the headquarters of the Combine empire on Earth as well as Dr. Breen's office. It also provided all of City 17's electricity via a Dark Energy generator, which was later destroyed by Gordon Freeman in an attempt to prevent the creation of a Superportal to the Combine homeworld.

Aperture Laboratories
Aperture Laboratories are the head-quarters of Aperture Science, another research organization hired by the US government at first to manufacture shower curtains. Located in Cleveland, Ohio, USA, the main building consists of an underground Enrichment Center, which is monitored and controlled by a powerful AI, GLaDOS. Preceding the death of their founder, the company started developing portal technology, most notably the Aperture Science Handheld Portal Device. The building was famously locked down by GLaDOS with employees still trapped inside. GLaDOS was later partially destroyed by a surviving test subject, but remains "alive".

Sometime before the Black Mesa Incident, Aperture Laboratories had a research vessel named the Borealis. Rumored to contain a top-secret project, but lacked several critical safety features due to lack of funds. It has gained legendary status ever since the entire ship, along with a large portion of its dry dock mysteriously vanished one night, never to be heard from again until it was found many years later by Judith Mossman, deep within a frozen crevice in the Arctic Circle.

The only connection between the Half-Life series and Portal are the Borealis in Half-Life 2: Episode Two and a slide show mentioning Black Mesa in Portal. Whether any further connections will be revealed in later installments is unknown.

White Forest
White Forest is an alpine missile base that was abandoned after the Cold War, and later purchased by Black Mesa in response to budgetary issues. The human Resistance later take over the site as a major base of operations, and use one of its rockets to stop a Superportal from opening up and preventing a massive Combine invasion.

After the Black Mesa Incident, the few remaining scientists escape to White Forest in an effort to protect themselves. Secure from the Combine tyranny and surrounding alien hazards, it is a haven for many survivors and resistance fighters.

List of appearances

 * Half-Life: Day One
 * 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
 * Portal ARG
 * Portal 2