Serial observations of individual submarine hydrothermal vents1 and the mapping of dilute hydrothermal plumes extending far downcurrent from vent fields2–4 indicate a stability of vent field fluid composition and mass flux on at least decadal time scales. The inherent episodicity of ridge-crest tectonic activity, however, suggests that discontinuous emissions of hydrothermal fluids also occurs. In support of this hypothesis we report here the discovery of a 700-m-thick, 20-km-diameter eddy-like 'megaplume' created by a brief but massive release of high-temperature hydrothermal fluids near 44°49' N, 130°14' W on the Juan de Fuca Ridge. The megaplume had a mean temperature anomaly of 0.12°C and overlay compositionally distinct plumes emanating from an apparently steady-state vent field at the same location. The megaplume was formed in a few days yet equalled the annual output of between 200 and 2,000 high-temperature chimneys.