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Earth is the third planet in the Sol planetary system, most notably inhabited by humans, and orbited by the Moon. After the Black Mesa Incident, the planet became overrun by hostile alien creatures from the Xen borderworld, and later taken over completely by the Combine.
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 traveling 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.
The resonance cascade that caused the Incident caught the attention of the Combine empire, who, after the detainment of Gordon Freeman by The G-Man and the destruction of the Black Mesa Research Facility began to send their own legions to Earth. A short but decisive battle between the world's military and the Combine ensued, later called the Seven Hour War, ultimately resulting in the enslavement of Earth's Human population.
Under Combine control, Earth was left in utter chaos. The Combine began to strip the Earth of any useful natural resources, the planet being "picked to the bone". The most noticeable sign of this was the considerable drop in sea levels as water was drained and brought to other Combine worlds. Toxic and radioactive waste is prevalent in many areas, making them inhospitable. Deleted material from Half-Life 2 included a Combine facility 'absorbing' the air while distributing a collection of chemicals into the atmosphere. Also, the introduction of fierce, extraterrestrial lifeforms such as the Antlions and Headcrabs left possibly all of Earth's natural lifeforms either extinct or critically endangered. Because of all of the above factors, even if Combine rule ends, extensive rebuilding, cleanup, and Xeno-extermination would have to take place to make the entire planet habitable again.
As seen on a globe in Half-Life: Alyx, a giant X is drawn over North America (except Alaska, though this may be a drawing error and it may be affected too given the size of the X), implying it to have been completely destroyed, or for some other reason uninhabitable after the Combine invasion. On the same map, giant question marks are drawn over Africa, Greenland, Oceania and Asia, indicating that the current status of those continents is unknown (at least to the drawer) after the invasion. Two large arrows are drawn leading to a large circle located in Eastern Europe from North America. This, along with the fact that most of the inhabitants of City 17 (which is located in Eastern Europe), are American/Canadian, suggests that the people living in City 17 were relocated (presumably by the Combine) there after their home continent was somehow negatively affected by the invasion.
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, sparking 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.
Interestingly, at the end of Portal 2, Chell looks upon the Earth after being nearly sucked out into space from the portal she opened on the Moon, witnessing that it is a healthy blue and green color, and later when she is released by Glados, she steps out into a vibrant wheat field during a clear sunny day. Given that Portal 2 takes place at an undetermined time after the first Portal, this may indicate that the Combine have been completely defeated and Earth has been restored.
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.
Santego Military Base
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 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 connections between the Half-Life and Portal series are the Borealis in Half-Life 2: Episode Two, a slide show mentioning Black Mesa in Portal, a mention of Black Mesa in Still Alive, and Cave Johnson, founder of Aperture Science, mentioning Black Mesa in a recorded announcement in Portal 2. Whether any further connections will be revealed in later installments is unknown. However, these references seem to indicate that Portal and the Half-Life series take place around the same time, with the notable exception of Portal 2, as Portal 2 takes place several years after the first one, with the first Portal taking place sometime during or after the Combine Invasion.
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 took 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 escaped 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.