  It has been argued that ultracompact objects, which possess light rings but
no horizons, may be unstable against gravitational perturbations. To test this
conjecture, we revisit the quasi-black hole solutions, a family of horizonless
spacetimes whose limit is the extremal Reissner-Nordstr\"om black hole. We find
a critical parameter at which the light rings just appear. We then calculate
the quasinormal modes of the quasi-black holes. Both the WKB result and the
numerical result show that long-live modes survive for the range where light
rings exist, indicating that horizonless spacetimes with light rings are
unstable. Our work provides a strong and explicit example that light rings
could be direct observational evidence for black holes.
