The World Navel, as the name implied, was the navel of a world.
A navel was the scar left behind after an infant was born and its umbilical cord fell away. While developing within its mother, the fetus could not eat with its mouth or breathe through its nose, and thus relied on an umbilical cord to draw nutrients from the mother. Once the fetus had fully developed and was born, the cord lost its purpose and fell away, forming the navel.
Wizards already knew that the process by which super-void matter transformed into a material world was immensely complex and slow. The evolution from void power into the most fundamental energy nebulae of material energy was itself a miracle, while the process by which vast, boundless nebulae condensed and collapsed into material worlds was even more incomparably intricate and profound. This was also the beginning of the birth of multifarious worlds, and the reason every world possessed different internal rules.
The reason the Wizard World had been able to enter the Heart of the World was the flaw in the Heart of the World.
Following that line of reasoning, in theory, Green could likewise enter the interior of the Earth Vein World's heart through this World Navel, which had already lost the protection of its will!
"Though this is only a conjecture, the source of those Original Sin Stones unquestionably points to the inside of the Heart of the World. Only mysterious rules like the fantasy-becomes-reality rules within the Heart of the Wizard World could create Original Sin Stones. Original Sin Stones could not only bear Annihilation Force, but were also the foundation upon which the Original Sin Sealing Technique gathered the Power of the Eight Wastes. Yet research into the Original Sin Sealing Technique revealed that the so-called