To Break the Pillars of Heavens
The Underworld is a plane of existence serving as the source of souls.
The Underworld is ruled spiritually; the material of the plane responds to the beliefs of its inhabitants. In this way, the plane can have features not normally allowed by physics, such as a large volume being contained in a smaller one.
The Underworld is spoken of as being “below” the World, but this is a symbolic truth, not a literal one; traveling from one to the other does not actually involve a change in elevation.