- “This world is older than any of you know, and contrary to popular mythology, it did not begin as a paradise. For untold eons demons walked the Earth. They made it their home, their... their Hell. But in time they lost their purchase on this reality. The way was made for mortal animals, for, for man. All that remains of the Old Ones are vestiges, certain magic, certain creatures”
- ―Rupert Giles[src]
Earth was a demon dimension which became populated primarily by humans after the Old Ones were banished. However, it remained a home to various supernatural creatures such as demons and vampires. Many human warriors, such as the Slayer, spend their lives protecting Earth from apocalyptic threats.
The name "Earth" refers to a specific planet within the dimension, but can also be applied to the dimension as a whole.
- “Either the Earth was an improvement, a step up, or it was a ghetto. Either way, it was on its own. The Seed, the source of all that magic, was the only thing powerful enough to keep it from bleeding back.”
Earth was born from the Seed of Wonder, the source of all magic. It originated as a world of demons, and may have been "spillover" from another, far more horrific dimension. The Seed brought the demons and magical energies through to Earth and kept them there, acting as a sort of "cork" in a bottle, so long as it remained in place at the Hellmouth which would eventually become Sunnydale, California.
The Fall of the Old Ones and the Dawn of Humanity
- “Yes. In the beginning, before the time of man, great beings walked the Earth. Untold power emanated from all quarters—the seeds of what would come to be known as good and evil. But the shadows stretched and became darkness, and the malevolent among us grew stronger. The earth became a demon realm. Those of us who had the will to resist left this place, but we remained ever-watchful.”
- ―Jasmine, one of the Powers That Be[src]
Higher beings such as the Powers That Be also existed on Earth at the beginning of time. However, eventually the Old Ones rose up as the dominant race and the Powers That Be fled to other dimensions. The Old Ones ruled Earth for an unknown period of time known as the Primordium Age. However, the Old Ones were eventually overthrown by a new race; humanity. Humans managed to restore the balance between good and evil and banish the Old Ones to other dimensions.
The End of Magic
The End of Magic was an event that occurred in 2006 when Buffy Summers destroyed the Seed of Wonder in order to break Earth's ties with other dimensions in order to prevent an oncoming army of demons. The destruction of the Seed created various changes faced by Earth's inhabitants, the most notable being the creation of zompires in place of newly sired vampires and the depowerment of all magical practitioners. This was eventually undone when a new Seed was created and aged to fruition, though this created a new breed of vampires.
Since Earth was made from the Seed of Wonder, it functioned as a conduit for magical energy. Individuals could draw this power without threat of addiction anywhere on the planet, whether it be indoors or outdoors, as everything and everyone in this dimension was connected to it. This type of magic was specifically used by a coven from devon.
The vampires of Earth were bound by two metaphysical laws; the first being their infirmity to the sun's rays which caused them to combust and the second being the inability to being captured in reflective surfaces. These laws functioned as a balancer to their human and demon sides because in other dimensions that did not abide by these laws, such as Pylea, the vampire's "game-face" manifested as a mindless beast, which was its demon half in its purest form, while when the vampire was not in "game-face" their human side was pronounced.
Certain destinations on Earth were located on hellmouths, areas where the walls between dimensions were worn thin, causing the those destinations to be subject to strange occurrences and demonic activity. Opening a hellmouth would be catastrophic and was often used in attempts to bring upon the apocalypse.
- Wesley: “No. No, no, no. The hot spot is cold. Certain geographical areas are rife with psychic energy. These areas tend to function as - dimensional hot spots, natural gateways between worlds. I'm guessing Caritas is one such spot. But the catch is...”
- Angel : “Oh god, got to have one of those...”
- Wesley: “Creating a portal tends to deplete a hot spot of its psychic energy.”
- — Wesley Wyndam-Pryce explains the nature of "hot spots".[src]
Hot spots were areas that contained a high concentration of psychic energy which could cause the creation of portals into other dimensions if the right spell were performed. Unlike Hellmouths, hot spots were only known to create portals and their range was relatively small, as Hellmouths could encompass entire towns while hot-spots could be found inside buildings. Creating portals caused the psychic energy to deplete, preventing new portals from being made until they recharged. When multiple entities entered a portal, they would separate, meaning two individuals could at could end up at different ends of the world they arrived in. This could be dealt with by placing the individuals in a metal enclosure, such as a car, that covered them from the front, sides and behind.