The structure of the Hakone hydrothermal system and geochemistry of thermal waters are described. The subsurface temperature map and the zonal distribution of thermal waters strongly suggest that thermal energy of the Hakone system is essentially supplied by dense volcanic steam rich in sodium chloride coming up through the volcanic conduit, from which subsurface streams of sodium chloride waters are derived. The seismic activity of Hakone mostly takes place at relatively shallow depths in the central part of the caldera. The chemical and physical properties of the dense steam are examined assuming that the phase transformation of water to steam is the major cause for volcanic earthquakes. The Cl-SO{sub 4} chemistry permits estimation of sodium chloride content of 0.5 to 1% in original dense steam responsible for sodium chloride waters. Thanks to the work of Sourirajan and Kennedy (1962) temperature pressure condition of volcanic dense steam at depths of 1 to 2 km below sea-level is estimated to be about 385 C and 230 bars, dissolving 0.5 to 1% of sodium chloride in steam. Below the depth of 4 km, earthquakes seldom occur, the hydrothermal system is saturated with solid sodium chloride, resulting in lowered vapor pressure. This implies that the permeation of meteoric water to the volcanic steam system mostly takes place at a depth less than 4 km. The analogy of hot eyes (centers of geothermal fields) and cold eyelids (surroundings of low temperature area) is emphasized for better understanding of hydrothermal systems
