Behind the scenesEdit
At least for the sake of Hades, the term Underworld, appears to denote that Hades is physically located under the earth, below the surface of Gloriana. It is said that all the rivers of the earth flow into Hades. It is possible that there are gates into the Underworld near every river.
The Underworld is also a region within Hades based on context of some information in QFG5.