Kambalny is the southernmost volcano in the Kamchatka Peninsula. After a long repose stage, on March 24, 2017, it produced a strong phreatic eruption, which ejected an ash cloud to the distance of up to 1000 km. In summer 2018, we deployed ten seismic stations in the vicinity of Kambalny for one year. Data from this network and the only permanent station located at ˜20 km from the volcano provided arrival times of P and S waves for this study. We used them to localize earthquakes and obtain the 3D distributions of velocities of the P (Vp) and S waves (Vs), also mapping the Vp/Vs ratio down to a depth of ˜10 km. We identify an anomaly of high Vp/Vs ratio in the depth interval of 710 km, and interpret it as a magma reservoir responsible for Holocene activity of Kambalny. This reservoir appears to be connected with the volcano edifice by a linear zone of high Vp/Vs ratio, which may represent a system of fractures originated during the eruption in March 2017 and served as a pathway for magma ascent. We propose that the interaction of hot magma with meteoric fluids in shallow layers caused active boiling and steam formation in a closed reservoir below the volcano. After exceeding a critical pressure, the steam escaped to the surface causing an explosive eruption. We also found evidence that geothermal fields located to the north and northwest of Kambalny might be fed from separate deep sources.