Earth is the homeworld of the human race. It is the primary setting for Doom II and Doom Eternal, and is referenced heavily in Doom 3 and Doom (2016). It also briefly appears in Doom and Doom: Annihilation.
Doom/Thy Flesh ConsumedDoom's original ending and follow up Thy Flesh Consumed take place upon Doom Guy's return to Earth via a teleporter (at the end of Infernal), where he discovers the remains of his pet Daisy outside of his home city. He fights his way through his hometown, to stop the initial invasion by Hell. Setting up Earth's total invasion in Doom II.
Doom IIEarth is where most of Doom II takes place. In this game, Earth has been invaded by the forces of Hell and most of its population has been destroyed. The player who has traveled back to Earth by drop pod after an invasion of Mars, battles the demon forces here in order to allow the survivors to escape the ruined planet. At the end of the game, the invasion is defeated, and the survivors can presumably resettle and start rebuilding the planet.
In Doom 3, Earth is never seen in the game itself, but is mentioned multiple times as being the center of human civilization, as well as being the ultimate target of Dr. Betruger and the demon forces. It is hinted that the Ancient Martians may have fled to Earth after the devastating war with Hell, and that humans may be their descendants. It is also mentioned that at some point in the past, the demons were actually on Earth but were defeated and driven into Hell, hence their desire to return and retake the planet.
Doom II RPG
The middle of Doom II RPG is set on Earth, in the East Coast UAC Facility. The three main characters of the game are teleported there after working their way through the Moon's Lunar Outpost.
Earth at the time of Doom 2016 is heavily populated and prone to energy shortages. Thus the discovery of Argent Energy on Mars (2016) was a huge boon, allowing nearly limitless energy potential. UAC (2016) was mega-corporation created by Samuel Hayden to mine Argent Energy as well as research archaeological sites in the dimensions beyond.