Atlantis

Atlantis was a continent that once existed in the Atlantic Ocean.

For ages, legends have recounted the stories of this once-forgotten civilization in various cultures around the world.

According to the theories presented by Operative-132, the underground civilization of Agartha descended from the survivors of Atlantis after catastrophe struck their people who were then living on the Atlantis continent.

An Ancient Catastrophe
According to Zerrath, up until 10,000 years ago (roughly 8000 BCE) the race that became the Agarthans had ruled the Earth and it's inhabitants from their continent of Atlantis. But their then-leader Zalanubar was experimenting on what was then known as the Sapphire of Power. In his attempts to weaponize the Sapphire itself, he inadvertently caused it to explode, releasing a massive amount of energy into the atmosphere. This caused a chain reaction that resulted in a worldwide calamity, eventually destroying the continent of Atantis.

The survivors fled into underground sanctuaries that eventually became known as Agartha. As time wore on and their numbers grew, they moved deeper and deeper into the Earth and came up to the surface far less frequently. Eventually they closed up their civilization and hid away from life on the surface, seeing the emerging human civilizations as a threat to their own existence.

Appearances
Atlantis has not actually been seen, only referred to.
 * Diabolical Tales: Part II - Vengeance of the Men From Within The Earth (mentioned only)
 * Diabolical Tales: Part III - Betrayal of the Men From Within The Earth (mentioned only)

Behind the Scenes
Atlantis is the name of a legendary continent situated somewhere in the Atlantic Ocean. It was mentioned in the stories of Plato, but has since been scientifically discounted.