  Ghost imaging is an unconventional optical imaging technique that
reconstructs the shape of an object combining the measurement of two signals:
one that interacted with the object, but without any spatial information, the
other containing spatial information, but that never interacted with the
object. Ghost imaging is a very flexible technique, that has been generalized
to the single-photon regime, to the time domain, to infrared and terahertz
frequencies, and many more conditions. Here we demonstrate that ghost imaging
can be performed without ever knowing the patterns illuminating the object, but
using patterns correlated with them, doesn't matter how weakly. As an
experimental proof we exploit the recently discovered correlation between the
reflected and transmitted light from a scattering layer, and reconstruct the
image of an object hidden behind a scattering layer using only the reflected
light, which never interacts with the object. This method opens new
perspectives for non-invasive imaging behind or within turbid media.
