Overview
Within the earth between Antarok and Ælphyne lies the Ur'Duun, comprised of a seemingly never-ending expanse of caverns, tunnels, mine shafts, and railways winding deep into the earth and connecting the worlds above and below. The Ur'Duun is host to the Metaphor of Geomena, associated with earth, arteries, and ingenuity.
History
The exact origin point of the Ur'duun is difficult to place, but it has served as a connecting bridge between the Planes of Antarok and Ælphyne for at least as long as Antarok has existed. Over such time many have come and gone, civilizations rising and falling as the peoples within fail to find peace. Redcap Dryad, Goblins and Trow were some of the first outsiders to brave the depths of the Ur'duun, joining the Plane's native Nagual spirits. Over time came the events that gave birth to the Dwarves and Gnomes, as detailed here
In the modern era, the Ur'Duun is a series of interconnected tunnels and vast caverns. To all who have not spent their lives there, it is considered an indescribable labyrinth. Furthermore, most life within would seek to end the ignorant and pillage what value they can from the corpse left behind. For this reason on top of Planar Sickness, it is generally ill advised to travel into the Ur'Duun at all from any other Plane unless one has an experienced guide on top of a method to stave off the sickness.
Life within the Ur'Duun
What life is like within the Ur'Duun varies wildly based on a wide array of variables, see the following links for more information: