The article presents the main stages of studying the mineral ferruginous waters of the first Russian spa «Marcial Waters», the beginning of the 300-year history of which is associated with the name of Peter I. It gives the results of studies of natural sources in the 18th and 19th centuries. Studies of the mineral waters during the exploration and exploitation of the field in the 20th century have shown that in the local area, wells open confined groundwater that has various mineralization and iron content and forms the characteristic hydrochemical type of mineral water called marcial. It is noted that the results of the long-term, but insufficiently regular and analytically comprehensive series of observations of the composition of an extremely unstable system of ferruginous waters do not allow one to unequivocally identify seasonal and long-term changes in the composition of the water. Regular isotope-chemical studies, which began in 2018, will make it possible to identify the patterns of formation of unique fresh high-iron waters.