  Whether or not system is unitary can be seen from the way it, if perturbed,
relaxes back to equilibrium. The relaxation of semiclassical black hole can be
described in terms of correlation function which exponentially decays with
time. In the momentum space it is represented by infinite set of complex poles
to be identified with the quasi-normal modes. This behavior is in sharp
contrast to the relaxation in unitary theory in finite volume: correlation
function of the perturbation in this case is quasi-periodic function of time
and, in general, is expected to show the Poincar\'e recurrences. In this paper
we demonstrate how restore unitarity in the BTZ black hole, the simplest
example of eternal black hole in finite volume. We start with reviewing the
relaxation in the semiclassical BTZ black hole and how this relaxation is
mirrored in the boundary conformal field theory as suggested by the AdS/CFT
correspondence. We analyze the sum over $SL(2,{\bf Z})$ images of the BTZ
space-time and suggest that it does not produce a quasi-periodic relaxation, as
one might have hoped, but results in correlation function which decays by power
law. We develop our earlier suggestion and consider a non-semiclassical
deformation of the BTZ space-time that has structure of wormhole connecting two
asymptotic regions semiclassically separated by horizon. The small deformation
parameter $\lambda$ is supposed to have non-perturbative origin to capture the
finite N behavior of the boundary theory. The discrete spectrum of perturbation
in the modified space-time is computed and is shown to determine the expected
unitary behavior: the corresponding time evolution is quasi-periodic with
hierarchy of large time scales $\ln 1/\lambda$ and $1/\lambda$ interpreted
respectively as the Heisenberg and Poincar\'e time scales in the system.
