The paper is devoted to the interpretation of published data on the geochemical anomaly detected in interstitial water within a thick (about 1 km) Pliocene—Quaternary marine sequence recovered by Hole 986 (ODP Leg 162). Salinity of interstitial water in the core from this hole decreases downward the section and the value at the bottom becomes 1.5 times lesser, relative to the normal marine value at the top. Nature of this geochemical anomaly remains enigmatic so far. We examine this issue under a more general scope of the origin of the inverse interstitial and fissure waters in sedimentary rocks. We support the opinion of hydrogeologists who attribute the super-low mineralization of these waters to the influx of high-temperature CO2-rich fluids into the sedimentary cover from magma sources in the basement. This inference is confirmed by not only the distribution of TDS content in the interstitial water, but also its alkalinity, Li content, and Na/Cl ratio in the hole core. The obtained results should be considered in the analysis of the nature of the Knipovich Ridge, as well as the type, and age of the Earth’s crust in the vicinity.