Discharge of volcanic gases in the marine environment can lead to local perturbations in ocean acidity, with consequences for biological communities and the potential for hazards related to depressurization and release of gases at the surface. Numerous hydrothermal vents in the crater of Kolumbo submarine volcano (Aegean Sea) are discharging virtually pure gaseous CO2 together with clear fluids at temperatures up to 220 °C. Acoustic imaging of the ascending bubbles suggests that the gas is being dissolved into seawater within ∼10 m above the crater floor (500 m below sea level). Dissolution of the gas likely causes local increases in water density that result in sequestration of CO2 within the enclosed crater, and the accumulation of acidic seawater. Lack of macrofauna at the Kolumbo hydrothermal vents, occurrence of carbonate-poor sediment in the crater, and pH values as low as 5.0 in recovered water samples point to acidic conditions within the crater. Buildup of CO2-rich water in the bowl-shaped crater of Kolumbo may be producing conditions analogous to some African volcanic lakes (Lake Monoun and Lake Nyos, Cameroon) where overturn of gas-rich bottom waters led to abrupt releases of CO2 at the surface. A minimum estimate of 2.0 × 105 m3 (STP) of excess CO2 may currently exist in the bottom 10 m of the Kolumbo crater.