An astronomer, science popularizer and writer who was enormously popular in his lifetime, Camille Flammarion (1842-1925) left a vast amount of personal papers archive that has been little studied until now. The ARCHIFLAMM project has two main aims: (1) to prepare a digital scientific edition of his correspondence and parts of his archives; and (2) to use this unpublished corpus to explore from a transnational and interdisciplinary perspective (history of science, cultural and social history, literary studies, anthropology) the foundations of the scholar's popularity on the margins of official science. ARCHIFLAMM will produce and stimulate new work in four interrelated areas of Flammarion's practice: (a) amateur science and observatory techniques; (b) epistemic and social dimensions of a transnational network; (c) literary techniques in popular scientific writings; (d) spiritualism and research into life after death and extraterrestrial life. ARCHIFLAMM relies on the collaboration of members of the Société astronomique de France (SAF), founded by Flammarion in 1887.