Abstract <p>The present paper reports the results from a study of hydrothermal activity in the young caldera of the active Zavaritsky Volcano (Simushir Island, the Kuriles) in 2023‒2024. Lake Biryuzovoe that fills the caldera is one of the 11 constant calderic and crater lakes on the Kuril Islands. The hydrothermal discharge was mapped from on board a drone bearing a thermal infrared camera. Hot springs are discharged into the lake on the southwestern shore for a length of 500 m, mostly below water line. The spring water has a chloride sodium composition with temperatures occasionally reaching 70°C, salinity 2.3 g/L, and pH 6.9, which is similar to the 1955 published data, that is, for an intereruptive period. Based on calculation of components in the lake water balance, the discharge of thermal waters is about 60 kg/s. Data from the literature were used to consider the variation in the physical and chemical characteristics of the lake and the southwestern springs. The 1957 eruption caused substantial changes in the southwestern springs and in the lake, both as concerns the anion and the cation part. The fresh-water cold lake was transformed into an ultra-acid one (pH 2.95) with temperature 34°C and the total chloride and sulfate ions equal to 2.4 g/L.</p>