The esoteric one (Agartha, Shambhala, hidden masters beneath the earth) is nineteenth-century occult geography. The pulp one is the Shaver Mystery from 1945, arguably the first mass-participation entity flap of the modern era. Underground is also where the fae live, which is not a coincidence so much as an old habit of mind.
The pulp strand generated genuine distress in its readership, and some of that correspondence describes people in real difficulty. A published story can produce the experiences it describes.
Ordinary bodies and ordinary evolutionary history. Not us, but never anywhere else either.
Degree of embodiment: 5, an ordinary biological body
Scale, 1 to 5
5 is an ordinary biological body. 4 is physical but manifests oddly. 3 comes and goes. 2 is energetic or subtle. 1 is consciousness with no persistent form. Note that this tends to fall as a tradition ages.
F: pre-modern folklore
Source code
Inherited oral tradition with no author and no publication date, in several cases continuous to the present.
D: disputed documents
Source code
Founded on texts of contested or admitted authorship rather than on witnesses.
The Shaver Mystery
From 1945
Richard Shaver's underground Deros and Teros, published as fact in a pulp magazine. Reader mail claiming corroboration arrived in volume: the clearest documented case of a published story generating its own witnesses.
Disputed or admitted authorship
Documented harm
The founding documents have contested authorship, or someone has claimed to have written them. Treat anything built on top as built on sand.
Deliberate contact work
Read this first
The hazard in this subject attaches to the method, not to which name you aim it at. Ouija work, invocation, sustained channelling, induced hypnagogia, deliberate sleep deprivation and hypnotic regression are the practices associated with difficulty, and a session aimed at a benevolent civilisation is not safer than one aimed at something considered lower. What raises risk is doing it alone, doing it repeatedly, doing it while sleep-deprived, and doing it with an existing vulnerability to psychosis, mania or dissociation. People who get into trouble here overwhelmingly got there through practice, not through reading.
Who should be careful
Practice-side
Anyone with a personal or family history of psychosis, bipolar disorder or dissociative conditions, anyone in acute crisis, and anyone isolated from people who would notice a change in them. Intensive contact practice is destabilising by design (that is what it is for), and destabilisation lands hardest on people already close to an edge.