  The problem of the event horizon in relativistic gravity is discussed.
Singular solutions in general relativity are well known. The Schwarschild
metric of a spherical mass is singular at zero ($r = 0$) and at the event
horizon ($r = r_g$). Both features reflect the existence of the phenomenon of
collapse in general relativity for compact masses exceeding $3M_{\odot }$. A
material particle crossing the event horizon falls into a central singularity
according to the classical theory of general relativity. In the quantum theory
of gravity, there may be no central singularity. The physics of the event
horizon is currently being refined. A promising technique is the study of
gravitational waves (GW) accompanying the merger of binary black holes at the
ringdown stage. GW observations of quasi-normal modes of the newly formed super
dense remnant will help clarify the physics of the event horizon.
