  Observations show that maximally-steep breaking water wave crest speeds are
much slower than expected. We report a wave-crest slowdown mechanism generic to
unsteady propagating deep water wave groups. Our fully nonlinear computations
show that just prior to reaching its maximum height, each wave crest slows down
significantly and either breaks at this reduced speed, or accelerates forward
unbroken. This finding is validated in our extensive laboratory and field
observations. This behavior appears to be generic to unsteady dispersive wave
groups in other natural systems.
