Xavier Remington 
The Underworld is considered an actual physical extension of the metaphysical higher dimensional Hell. The horrifying home of former fallen angels who became demons under the command of Satan The Devil. If one were to venture deep enough into the brimstone bowels of the Underworld they’d gradually transmute into a metaphysical energy form as you entered the fifth dimension. It’s one of the few ways for anyone to directly enter a higher dimension realm without any paranormal powers or advanced technology. The Underworld is a sinister series of caverns that literally wind down from the Earth’s surface into the molten core of the planet. This is the point the gradual metaphysical energy transformation is finalized. It’s known as Tartarus. The place physical, and metaphysical flames of damnation meet to create exquisite unimaginable torment! It is considered the central gateway to Hell itself!
The nature of the rock that composes the caverns is of a metaphysical matter form. This, and the sheer insane depth make the Underworld undetectable by humankind’s technology thus far. There are a number of hidden cave entrances around the world that can only be found by following the clues in various ancient texts. It’s easy enough to get in but escaping is far more difficult! More often it’s spirits that break free rather than monstrous physical entities!
The physical structure of the Underworld first came into existence 252 million years ago. It’s a material manifestation of the angst ridden bowels of Goddess Gaia Mother Nature. Something that occurred when her spontaneous birth as Earth’s soul caused a mass extinction! However she quickly corrected Gods evolution program in her own unique way. Some millions of years later the Devil, and his demons came to Earth amid the age of dinosaurs. They began tinkering with the evolution process adding paranormal elements that included the birth of purely physical low level demons. They even created their own race of reptilian humanoids. The infamous meteor hit that caused the extinction of the dinosaurs 66 million years ago was a first volley in a fight to drive the demons, and their creations underground into the Underworld. An act to make way for the creation of humanity. The Devil, and his original fallen Angels, went back to their first stop on the fall from Heaven. That being their five to six dimensional spiritual Hell. They then created the direct connection with the Underworld. The rest of the demonic creatures began carving out a life in the Underworld. However some would escape every so often.
With each passing epoch of pre-history more nature Gods, and Goddesses came into existence. Some falling toward the force of darkness that drew them to the Underworld. They generally stuck to the upper levels of the caverns, and were only partially welcomed by the low level demons that resided much deeper. As human civilization took shape a major explosion of deities came into existence via the power of the human collective consciousness. A greater number of dark Underworld Gods, and Goddesses flocked forth from cultures around the world. Quite a few claiming human souls as their own before that of Heaven. In essence a hijacking of the natural afterlife process. Eventually the Demons, and Earthly Gods went to war over control of the Underworld. It seemed the demons were to win until the Archangels of Heaven agreed to lend a one time hand in exchange for allowing the majority of human souls to pass into the Heavenly afterlife.
Even after the dark Gods had command of most of the Underworld, the various factions fought among themselves. Eventually they carved up sections for each other, and respected the boundaries via supernatural treaties. This included Greek, Roman, Egyptian, Celtic, Chinese, and a plethora of other deities worshiped by their respective cultures. In most cases they welcomed the terrifying creatures that lie within including Dragons. Even various demons were allowed to work among the Gods. Particularly if they shared similar dark goals.
Currently the Greek Gods of Mount Olympus, under Zeus ,are the strongest faction of Gods. They are partnered with a number of other 
Virtually every planet that had, has, or will have intelligent life contains its own version of the Underworld. After All the Devil, and his demons are higher dimensional beings that can be on multiple worlds, and Universes at the same time. Some connect to the metaphysical Hell while others may not. Particularly on worlds where the forces of good have completely dominated things!
Interesting Note: The Hollow Earth Theory has its true origins in the Underworld. The famous Hang Son Doong cave is a cut off uppermost layer of the Underworld. Video Of The Giant Cave.
The Devilish Voices Of The Underworld
Pockets of Sulfur Hexafluoride gas flourish in the deeper zones of the Underworld thus causing some of it’s entities to have rather evil sounding voices! As you near Hell itself Red Noise can be heard as well.
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If someone stayed in the underworld, would their Metaphysical Matter form not age or decay granting them a immortality?
Yes while in the Underworld one is immortal. Someone could theoretically live down there for thousands of years, and live to see a future they normally would have been dust for!