Franck Goddio presents his research on the remains of a large religious complex in the Portus Magnus of Alexandria. The existence of a temple to Poseidon is mentioned by Strabo in his description of the Portus Magnus of Alexandria: “Next to the Poseidium, which is an arm projecting from the so-called Emporion, on which is a temple of Poseidon…” (Geogr. 17, 1,9). In this lecture Franck Goddio presents the research of the European Institute for Underwater Archaeology (IEASM) in the Portus Magnus and on this “arm” of land which stands out clearly on the topographical map obtained. At the base of this peninsula, the remains of a large monument are still visible, partially sticking out of the sediment. Limestone ashlars, pink granite blocks, fragments of pink granite columns, and imposing mortar blocks are scattered on an area of over 3,000 square meters. The study of the monument’s environment reveals the existence of a large religious complex giving rise to an ambitious urban planning concept. This lecture was recorded as part of the 2025 conference "Alexandria and the Sea II" held by the Oxford Centre for Maritime Archaeology at Harris Manchester College at the University of Oxford. For more information about the OCMA: https://ocma.web.ox.ac.uk/ For more information about the European Institute for Underwater Archaeology (IEASM): https://www.ieasm.org/ https://www.franckgoddio.org/ https://www.youtube.com/@Franck_Goddio For more information about the Hilti Foundation: https://www.hiltifoundation.org/