Significance The dissociation of water is arguably the most fundamental chemical reaction occurring in the aqueous phase. Despite that the splitting of a water molecule very seldom occurs, the reaction is of major importance in many areas of chemistry and biology. Direct experimental probing of the event is still impossible and also simulating the event via accurate computer simulations is challenging. Here, we achieved the latter via specialized rare-event algorithms estimating rates of dissociation in agreement with indirect experimental measurements. Even more interestingly, by a rigorous analysis of our results we identified anomalies in the water structure that act as initiators of the reaction, a finding that suggests paradigms for steering and catalyzing chemical reactions.