The Underworld is considered an actual physical extension of the metaphysical higher dimensional Hell. The horrifying home of demons. If one were to venture deep enough into the brimstone bowels of the Underworld they’d eventually transmute into a metaphysical form. It’s one of the few ways to directly enter a higher dimension. 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 Godly factions around the world in preparation for the War Of Armageddon. Zeus has been the most charismatic, and prolific God in marshalling paranormal power despite the deeply dwindled worship among humans. His Dark God Hades is the current King Of The Underworld with Persephone acting as the Dark Queen. The Egyptian God Osiris is the Dark Prince Of The Underworld. The Norse Goddess Hel, Daughter of the notorious God Loki, is the Dark Princess Of The Underworld. These positions are based on how much power each deity wields rather than any type of voting process. In order to prevent further conflicts with the Demons Of Hell, the Devil holds the title Emperor Of The Underworld just as he is the Emperor of Hell. Although he rarely exercises that title, and simply only asks that the Underworld allow free unimpeded access to demons. He realizes that keeping the peace is necessary so he can direct his dark forces on ventures that will lead him to victory in the upcoming battles of Armageddon.
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.