Earth is the third planet in the Solar System, and homeworld of the human race. It is the primary setting for Doom II, and it is referenced heavily in Doom 3. The corporate headquarters of the Union Aerospace Corporation are located there. In all versions of the Doom canon thus far, humans have achieved proficiency in interplanetary space flight and have begun to spread out from Earth to colonize and exploit the resources of neighboring worlds and their moons. This is supported with a network of spaceports on Earth, as well as stations and platforms in orbit around it.
Earth is where most of Doom II takes place. In the game, Earth has been invaded by the forces of Hell which were sent out by the spider mastermind at the end of Doom, and most of its population has been destroyed. The player battles the demonic forces there in order to allow the survivors to escape the ruined planet via space ship. As on the Martian moons of Phobos and Deimos, portions of the Earth begin to resemble Hell to a greater degree as one progresses toward the source of the corruption. Though the marine at one point passes what seems to be a portal to Hell, the areas on the other side still resemble Earthly locations that have been subsumed into the other reality. At the end of the game, the invasion is defeated, and the survivors can presumably resettle and start rebuilding the planet.
In the Final Doom mission The Plutonia Experiment, remnants of the UAC are reorganized by the government with the aim of carrying out research toward preventing another such catastrophic supernatural invasion. As part of this, gateway technology is first relearned, and a prototype device which can close the gates known as the quantum accelerator device is then developed. While initial tests prove successful, a massive attack from the other side manages to overwhelm the defenses setup around the experimental gateways, leaving Earth vulnerable once again and the prototype QAD stolen by the demons, leading to fears it would be used in some horrific fashion against humanity. The marine is again called into action to reclaim the QAD and stop the demons.
No Rest for the Living
In the No Rest for the Living expansion, taking place immediately after Doom II, a surviving cyberdemon lord has setup his own "personal amusement park" on Earth, leading a large number of surviving demons to attack the Earth Base and neighboring facilities. As part of his reconstruction efforts, the marine must make his way to the cyberdemon by cutting through his Hellish hordes and destroy him.
In Doom 3, Earth is never seen in the game itself, but it is mentioned multiple times as being the center of human civilization, as well as being the ultimate target of Dr. Betruger and the demonic forces. It is hinted that the ancient Martians may have fled to Earth after their devastating war with Hell, and that humans may be their descendants. It is also mentioned that at some point in the past, the demons had dominion over the Earth, but were defeated and driven into Hell, hence their desire to return and retake the planet.
The player arrives at Mars aboard the interplanetary dropship Darkstar after departing a spaceport at Burns Flat, Oklahoma, three months prior, along with UAC attorney Elliot Swann and his bodyguard Jack Campbell, who were sent to investigate Betruger's activity after prominent UAC scientist Dr. Elizabeth McNeil blew the whistle on his suspicious teleportation research program. A devastating invasion of the Mars base follows mere minutes later.
Various PDAs throughout the game contain emails from employees to or from their family and friends back on Earth. Interplanetary marketing company Martian Buddy frequently transfers goods there from Earth. A form of subspace communication technology is referenced in a news broadcast which apparently allows faster-than-light communication between the two planets.