  The merger of a binary black hole gives birth to a highly distorted final
black hole. The gravitational radiation emitted as this black hole relaxes
presents us with the unique opportunity to probe extreme gravity and its
connection with the dynamics of the black hole horizon. Using numerical
relativity simulations, we demonstrate a connection between a concrete
observable feature in the gravitational waves and geometrical features on the
dynamical apparent horizon of the final black hole. Specifically, we show how
the line-of-sight passage of a "cusp"-like defect on the horizon of the final
black hole correlates with "chirp"-like frequency peaks in the post-merger
gravitational-waves. These post-merger chirps should be observed and analyzed
as the sensitivity of LIGO and Virgo increases and as future generation
detectors, such as LISA and the Einstein Telescope, become operational.
