This article is from the Religion-Shamanism FAQ, by Dean Edwards, Stef Jones, Jilara, Jane Beckman, Ann Albers with numerous contributions by others.
These may be lower parallel
regions which are otherwise similar to this world or dark, shadowy
realms or hells. In the case of such regions as the Celtic Underworld,
these lower Earths may not have a sun to produce light, but rely on
light which is naturally emitted by the land itself.
Dean Edwards (firstname.lastname@example.org)
"Lower World" is a term used by Harner and associates to refer to the
part of non-ordinary reality that one reaches by journeying through a
tunnel. Usually power animals and wisdom about the body and physical
aspects of existence can be found in the lower world.
Stef Jones (email@example.com)