In situ observations of submarine hydrothermal activity have been conducted in Punta Banda. Baja Califronia, Mexico, approximately 400 m from the coast and at a seawater depth of 30 m. The hydrothermal activity occurs within the Agua Blanca Fault, a major transverse structure of Northern Baja California. Hot springwater samples have been collected and analyzed. Marked differences exist between the submarine hot springwater, local land hot springwaters, groundwater, and local seawater. SiO2, HCO3, Ca, K, Li, B, Ba, Rb, Fe, Mn, As, and Zn are enriched in the submarine hot springwater, while Cl, Na, So42, Mg, Cu, Ni, Cd, Cr, and perhaps Pb are depleted in relation to average and local seawater values. Very high temperatures, at the hydrothermal vents, have been recorded (102¿C at 4-atm pressure). Visible gaseous emanations rich in CH4 and N2 coexist with the hydrothermal solutions. Metalliferous deposits, pyrite, have been encountered with high concentrations of Fe, S, Si, Al, Mn, Ca, and the volatile elements As, Hg, Sb, and Tl, X ray dispersive spectrometry (1500-ppm detection limit). X ray diffraction, and scanning electron microscopy of the isolated metalliferous precipitates indicate that the principal products of precipitation are pyrite and gypsum accompanied by minor amounts of amorphous material containing Si and Al. Chemical analyses and XRD of the reference control rocks of the locality (volcanics) versus the hydrothermally altered rocks indicate that high-temperature and high-pressure water-rock interactions can in part explain the water chemistry characteristics of the submarine hydrothermal waters. Tritium dating of the hydrothermal solutions places them in the prebomb period of 1953 with an undetermined old age. Their long residence time, the occurrence of an extensive marine sedimentary formation, their association with CH4 and their similarities with connate waters of oil and gas fields suggest that another component of their genesis could be in cation exchange reactions within deeply buried sediments of marine origin. The approximate volume discharge of the hydrothermal system has been measured (330,000 m3/yr), and the overall convective heat flux over the mapped submarine hydrothermal area has been calculated, the value of which exceeds 105 times the reported average conductive flux through the sea floor. Our data and the observations indicate that metal-rich marine sediments and marine ore-forming processes are a concurrent reality and that ores are being formed today in submarine areas of high convective heat flow where hydrothermal activity is the precursor for their occurrence. At the same time the results of our investigations have demonstrated that submarine hydrothermal activity and submarine hot springs exist in the ocean in regions characterized by relative quiescence and that tectonically 'active' ridge environments are not an exclusive prerequisite for their existence.