This paper reports the first experimental results in which the kinetic energy of cold fog, generated in a water arc plasma, exceeds the electrical energy supplied to form and maintain the arc. The cold fog explosion is produced by breaking down a small quantity of liquid water and passing a kiloampere current pulse through the plasma. The 90-year history of unusually strong water arc explosions is reviewed. Experimental observations leave little doubt that internal water energy is being liberated by the sudden electrodynamic conversion of about one-third of the water to dense fog. High-speed photography reveals that the fog expels itself from the water at supersonic velocities. The loss of intermolecular bond energy in the conversion from liquid to fog must be the source of the explosion energy.