Redox-sensitive metallic elements, Mn and Fe, are oxidized in deep sea waters and form abundant ferromanganese crusts and nodules on the world's ocean floors at ultraslow rates of growth. This process of oxidation and the mechanism of precipitation are yet unknown. In this paper, the results of the first successful, long-term, on-site experiment of mineral precipitation that ascertains modern, ongoing hydrogenetic deposition of oxide materials from normal seawaters at water depths of 900-4500 m of geologically active and inactive environments are presented. We succeeded in the in-situ precipitation experiment on the sea floor and characterized the precipitates using high-resolution and submicron-scale chemical, mineralogical, and structural analyses. The installed artificial plates of glass, ceramics, and plastic yielded spread-out particles of sizes varying from one to a few micrometers in diameter, of coccoid-like irregular shapes, with a maximum of 1,000-10,000 individual particles/mm2/year after 12-15 years of exposure. The results indicated a continuous substantial growth of the hydrogenetic minerals if both Mn and Fe are supplied to the bottom waters. The mineralogical, chemical, and structural properties of the precipitates are similar to those of the natural precipitates on the seabed that are made up of hydrogenetic ferromanganese crusts and nodules, together with settling sediments, suspended hydrothermal particles, or microbial precipitates from cultivated Mn-oxidizing bacteria. Our work presents new realistic insight into proposed genetic models of marine hydrogenetic ferromanganese deposits in modern diverse ocean environments.