Eternity Peak, Eternal Barrier, and Eternal Corridor.
According to historical recordings, those three structures seemed to appear around the same time the Buried City was established. Transcripts only briefly documented that the marvels were built by the greatest and most respectable artisans of the Underearth race. The Buried City was definitely the grandest and most unbelievable man-made megastructure throughout the entire Mythical Continent.
However, up to that point, not many could claim that they understood the Buried City thoroughly because more and more secrets were found or rumored to be hidden within the giant establishment.
Not only did the well-known Elders on the Mythical Continent not know much about the Underworld, but they did not have a lot of information regarding the locked-down Buried City too.