This book is an experiential guide to the spiritual realities of alchemy. The techniques revealed are based on centuries of work by philosophers of nature who sought universal laws of transformation that were as valid in their laboratory experiments as they were in their own hearts and minds. The secret teaching of their "Underground River" has flowed as a legacy of wisdom from one generation of alchemists to the next. Collectively, they sought to create a spiritual technology that worked on all levels at once, a powerful Science of Soul whose principles are as valid in the physical world as they are in the psychological and spiritual realms.