  The nuclear option at THERA provides an ideal and unique opportunity to
investigate the black body limit (BBL) of high energy Deep Inelastic Scattering
of highly virtual photons off heavy nuclear targets and thereby probe QCD in a
new regime. At high enough energies, whichever hadronic configuration the
photon fluctuates into, the interaction at a given impact parameter with the
heavy nuclear target will eventually reach its geometrical limit corresponding
to the scattering from a black disk. An attractive feature of the BBL regime
for a large nucleus is that the interaction is strong although the coupling
constant is small. Predictions for longitudinal and transverse distributions of
the leading hadrons in inclusive and diffractive channels and exclusive vector
meson production are found to be strikingly different in BBL and the leading
twist approximation. In particular, the multiplicity of leading hadrons in the
current fragmentation region is strongly suppressed, while the cross section of
diffractive vector meson production by longitudinally polarized photons is $
\propto 1/Q^2$. The x and $Q^2$ ranges where BBL maybe reached are calculated
for the interaction of color triplet and color octet dipoles at central impact
parameters. We conclude that for heavy nuclei THERA would reach deep into BBL
for a wide range of $Q^2$. The connection between hard diffraction and leading
twist nuclear shadowing is analysed. It is demonstrated that the current
leading twist analyses of the HERA diffractive data lead to similar and very
large leading twist shadowing for gluons. Several processes sensitive to the
amount of nuclear shadowing for gluons are discussed.
