There are many visions of the Underworld that are available through every major religion and culture. Called by many names, Sheol, Hell, Inferno, Hades, Xibalba, just to name a few, the Underworld is part of the human collective unconscious. Its descriptions range from a burning land of torture to an alternate reality for the dead where both rewards and punishment are meted out. For many the Underworld is simply the farthest one can be from God, home to demons and the soulless.
The Underworld is a place of the unconscious. It is a place where we store the darker, less acceptable pieces of our Selves. Who or What labels us ‘dark’ or ‘unacceptable’? Society tells us what masks to wear and how to act to be accepted. Family members, lovers, people in authority may label versions of us ‘unworthy’ during our lifetimes. These parts of our Selves we push away and deny and ultimately condemn to the Underworld.
The Underworld in my experience is also a place which exists in another dimension. I dare say it is a dimension of its own. There are portals where this dimension bleeds through to our own, some of these portals are fixed, a few are manually created and opened for a period of time, while others fluctuate. These are portals to our reality, our dimension. These portals allow us to energetically enter the Underworld and conversely allow the dwellers of the Underworld to cross over to our dimension. We know them as demons primarily. Their ability to manifest [or become visible, or effect physical objects] is dependent upon their inherent power, proximity to the portal and the sustenance they can find.