  Gravitational waves are rapidly becoming a very reliable tool for testing
alternative theories of gravity. In particular, features in the gravitational
wave emission during black hole ringdown phase provide a direct probe of the
spacetime outside the black hole. In this article, we consider black holes in
ghost-free massive gravity. These black holes generically have scalar hair. We
found that a simple coupling between gravitational perturbations and this
scalar hair can change the quasinormal ringing of the black hole, and in
particular, produce echoes in the emitted gravitational waves. This finding
provides a clear-cut way to test massive gravity theory using gravitational
wave observations.
