Active hydrothermal vents have been discovered on the East Pacific Rise at 21°N . The most spectacular of the vents jet out 350°C water at flow rates of several meters per second. The heat loss associated with a single vent of this type is three to six times the total theoretical heat loss for a 1-km segment of ridge out to 1 m.y. age. This underscores the importance of hydrothermal circulation in the heat budget of mid-ocean ridges. It also requires that vent activity of this type be highly episodic rather than steady state.