The potential behind the horizon of a black hole in classical theories is
described in terms of data that is available to an external observer - the
reflection coefficient of a wave that scatter on the black hole. In GR and
perturbative (in α' ) string theory the potential is regular at the horizon
and it blows up at the singularity. However, the exact reflection
coefficient, that is known for the SL(2,R})/U(1) black hole and includes
non-perturbative α' effects, seems to imply that there is a highly
non-trivial structure just behind the horizon.