This book examines what we know about the inhabitants of the Invisible World, also known as the Otherworld or the Spirit World, depending on the culture. The author explores not only the various representations of the Invisible World in different cultures, but also their inhabitants. Who are they? What do they do? What do they say? These are important questions that need to be answered. The author questions a number of researchers and specialists in shamanism, folklore, psychology, prehistory, the UFO phenomenon and the science of religion in order to draw up as exhaustive a picture as possible. The role of psychotropic substances and shamanic trance are also addressed. In the final part of the book, the author sets out a project to explore the Invisible World in order to make contact with its inhabitants - an Experimental Anthropology - involving a team of explorers, the ethnonauts, specially trained for this type of investigation. Finally, having begun to explore extra-planetary space and the depths of the oceans, we have yet to discover a third, virtually unknown medium, the third frontier: the Invisible World... Michel Nachez has a doctorate in Ethnology and is a specialist in non-ordinary states of consciousness and virtual communities in cyberspace. He taught for 20 years at the Institute of Ethnology in Strasbourg. He directed a seminar on trance there. He has also taught in Education Sciences and at the École Supérieure d'Informatique pour l'Industrie et l'Entreprise. Since the 1970s he has been a researcher in the field of paranormal phenomena, spirit apparitions, shamanism and contact with ‘aliens’. Key words: shamanism, invisible world, spirits, UFOs, extraterrestrials, out-of-body experience, lucid dreaming, trance, ethnology, psychotropic drugs, DMT, Machine Elves, Fairy Folk, cave paintings