  We show the existence of superradiant modes of massive scalar fields
propagating in BTZ black holes when certain Robin boundary conditions, which
never include the commonly considered Dirichlet boundary conditions, are
imposed at spatial infinity. These superradiant modes are defined as those
solutions whose energy flux across the horizon is towards the exterior region.
Differently from rotating, asymptotically flat black holes, we obtain that not
all modes which grow up exponentially in time are superradiant; for some of
these, the growth is sourced by a bulk instability of AdS3, triggered by the
scalar field with Robin boundary conditions, rather than by energy extraction
from the BTZ black hole. Thus, this setup provides an example wherein Bosonic
modes with low frequency are pumping energy into, rather than extracting energy
from, a rotating black hole.
