Actually quotan the Mage core:
It is said that within the Shadow Realm there are caves or
holes in the ground that, were one to follow them far enough
— usually beyond all hope of return — they would lead to a
gray realm of eternal stillness. The Underworld.
Those entering the Underworld first come to a hallway
that appears to be endless, with many doors along its walls.
It resembles a mausoleum to some, a mansion to others, or
even a rough cave to others. Traveling to the Underworld is
the work of masters of Death. Lesser mages would be foolish
to attempt it. It is said that the shades there can fool men into
staying forever, even making them think they have escaped
back into the living world, while their bodies starve to death
in the dark pit.
Even Moros mages fear the Underworld. Their power
comes from Stygia, the Supernal Realm where death is a
lesson and balm for a weary soul, a place of waiting while the
soul prepares for the next stage in its journey. The Underworld
is a mistake, a casualty of the Abyss, where death no
longer rises to the Supernal and instead draws things down,
ever down, to lower worlds of non-being.
Nonetheless, there are reasons to go there, for it is said that
the ghosts of even Atlantean mages of old can be found there,
if one is lucky enough or has an Artifact that provides a
sympathetic connection. It is also rumored that the ruins of
Atlantis itself exist there, a shade of the city’s former glory
but still providing bold mages the opportunity for plunder
unequaled in the living world.