As the material plane merges with the planes of Near Earth, series of caverns and tunnels extend deep into the earth. Few have travelled there and come back to tell the tale; those few would tell you that beneath the earth there is a great prison city, filled with unimaginable things rarely glimpsed before. In the Near Earth, you will find many things never seen in the material plane, duplicates of strange forms found only in the Far Earth. It's a very effective prison; anyone who moves between the Near Earth and the material plane either explodes or undergoes tremendous amounts of horrible change, creating a generally unrecognizable abomination.
This is a prison where people (and things) are sent to be forgotten. There are no locks or keys; only the nearly endless boundary between material plane on one side, and the endless, shifting transition to the Far Earth on the other. The current incarnation was finished by a near-godlike wizard by the name of Gormloch in 1705 7th.
Any strongly earth aligned spirit who crosses the boundary to the material plane is in serious danger of spontaneous combustion. The highly ordered, highly differentiated energies of earth cannot handle the increase in entropic and other energies native to the material plane, and most of the time they suddenly lose their structure and burst into a million pieces. With larger denizens this sometimes causes major earthquakes and other subterranean phenomena.
Those that survive change and change and change. The adjustment process is much like sitting in a swirling magical storm for several minutes, and depending on how earth aligned the being is, they may undergo as many as a thousand changes in a single instant. This rarely leaves more than a ravenous hulk, hunting the underdark for food, driven by a rage it cannot understand.
The other logical exits, the boundaries with the planes of fire, water, light, and dark, are protected by vast walls constructed of a magical, transparent material as hard as steel and as smooth as silk. It is formed of nearly pure wind energy, and thus the denizens of the prison are effectively powerless to climb, break, or alter the walls in any way.
The wizard Gormloch died many centuries ago, and few use his prison now. Sometimes enemies of earth will be infused with large quantities of earth energy, bound to their bodies and souls, and cast into the city forever. Sometimes portions of the Far Earth wander across the border into the Near Earth, where they assume identity, and can never return to the Far Earth. More often, banishment spells granted by Earth Gods will send hapless individuals to this horrifying place, from which few ever return.