Oceanus is an enormous freshwater system that encircles the entirety of the Underworld and is also inhabited by the Titan of the same name. This subterranean source feeds all of the Earth’s freshwaters: lakes, rivers, and streams, as well as the rain and clouds.
On the farthest shores of the Oceanus is the faint smudge of shoreline that only the deities of the Underworld may set foot on without effort. Mortals and other divine beings will be blocked from entry by a magical force.
If, by chance, someone other than an Underworld deity does manage to cross Oceanus and break through the barrier, they will be greeted by the towering Triple Wall of Bronze. The only guaranteed path through the Triple Wall of Bronze is the Gate of Horn & Ivory, Gate of Dusk, Gate of Shades, or the Gate of Dawn. There are, however, rumours that there are numerous other entrances throughout the area that were built by unknown entities.
Depending on the visitor’s approach, they might find themselves in the Oneiroi, the Onyx Towers, the Land of Hekate, or in Elysium. However, this requires crossing one or more of the rivers that wind their way throughout the Underworld, and these rivers are typically impassable.