  We observed the classical interacting galaxy M51 with FAST and obtain high
sensitivity HI image with column density down to 3.8 $\times$ 10$^{18}$
cm$^{-2}$. In the image we can see a diffuse extended envelope around the
system and several new tidal features. We also get a deeper look at M51b's
probable gas, which has an approximated velocity range of 560 to 740 km
s$^{-1}$ and a flux of 7.5 Jy km s$^{-1}$. Compared to the VLA image, we
observe more complete structures of the Southeast Tail, Northeast Cloud and
Northwest Plume, as well as new features of the Northwest Cloud and Southwest
Plume. M51's most prominent tidal feature, the Southeast Tail, looks very long
and broad, in addition with two small detached clouds at the periphery. Due to
the presence of optical and simulated counterparts, the Northwest cloud appears
to be the tail of M51a, while the Northwest Plume is more likely a tidal tail
of M51b. The large mass of the Northwest Plume suggests that M51b may have been
as gas-rich as M51a before the interaction. In addition, the formation process
of the Northeast Cloud and Southwest Plume is obscured by the lack of optical
and simulated counterparts. These novel tidal features, together with M51b's
probable gas, will inspire future simulations and provide a deeper
understanding of the evolution of this interacting system.
