The serpent power said to sleep at the spine's base.
Documented
Documented practice, documented effects
Kundalini doctrine is detailed in tantric texts from the first millennium onward. Modern clinical literature documents kundalini syndrome: practitioners, and sometimes non-practitioners, reporting exactly the described sensations, occasionally requiring psychiatric care.
Assessment: The texts are documented and the experiences are documented. The serpent is the contested part.
Stories
Do not wake it alone
The warning every tradition repeats: premature awakening breaks people, and the snake should only be raised under a teacher, or, in the yoga-studio version, under no circumstances discussed at all.
Assessment: Documented as consistent instruction across lineages. The clinical case reports of meditation-related crisis give the warning an unusually practical basis for this shelf.
Interesting claims
One neurology, many costumes
The reductionist reading: kundalini, the Holy Spirit's fire, the vapor trances of Daoists, and various shamanic heats are one autonomic phenomenon wearing local costumes, and the tantrics simply wrote the best manual.
Assessment: Plausible, unfalsifiable, and rude to everyone equally, which is why it pleases no tradition. The archive notes that even if true, it would make the manuals more remarkable, not less.
Sources and further reading
Svatmarama, Hatha Yoga Pradipika, fifteenth century
Sannella, The Kundalini Experience (1987)
Scotton et al., Textbook of Transpersonal Psychiatry (1996)
Worth asking
If every tradition feels the same snake, is the snake real or is the spine?