Having traversed The Earth, Iapetus arrived in the Underworld.
Although, strictly speaking, the Underworld at this time is also a part of The Earth, it feels like a world unto itself, perhaps because it is tightly enveloped by the four Primordial Gods.
No matter where one enters the Underworld, whether from beneath the sea or the center of The Earth, unless they leap through dimensions like 'spirit realm traversal', everyone first appears at the edge of the Underworld, a place later guarded by the three-headed hound Cerberus.
Behind the gate lies the River of Sorrow, Acheron, with the Land of Eternal Night and the Realm of Lightness on either side. For anyone wishing to enter the Underworld, crossing the river is the only way.
Of course, at this time, the Underworld had neither gates, nor had the River of Sorrow come into existence.
