The construction of the marathon dam and the creation of the capital’s water supply network (1926) was one of the pioneering projects of the 1920s on a global scale. ULEN, the American company that undertook the project, decided to capture the process on a 35mm film. For years, the archival material remained untapped in the warehouses of the Water Company. In 2019, on the occasion of the 90 years anniversary since the construction of the dam, EYDAP (the Athens Water Supply and Sewerage Company) decided to process this material shedding light to a unique historical document. The film depicts the construction phases of the Marathon Dam, the Bogiati tunnel, the Galatsi Refineries and the Water Supply Network of Athens, Piraeus and Environs (1926-1931).