  We report the first high-sensitivity HI observation toward the spiral galaxy
M94 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). From
these observations, we discovered that M94 has a very extended HI disk, twice
larger than that observed by THINGS, which is accompanied by an HI filament and
seven HVCs (high velocity clouds) at different distances. The projected
distances of these clouds and filament are less than 50 kpc from the galactic
center. We measured a total integrated flux (including all clouds/filament) of
127.3 ($\pm$1) Jy km s$^{-1}$, corresponding to a H I mass of
(6.51$\pm$0.06)$\times$10$^{8}$M$_{\odot}$, which is 63.0% more than that
observed by THINGS. By comparing numerical simulations with the HI maps and the
optical morphology of M94, we suggest that M94 is likely a remnant of a major
merger of two galaxies, and the HVCs and HI filament could be the tidal
features originated from the first collision of the merger happened about 5 Gyr
ago. Furthermore, we found a seemingly isolated HI cloud at a projection
distance of 109 kpc without any optical counterpart detected. We discussed the
possibilities of the origin of this cloud, such as dark dwarf galaxy and RELHIC
(REionization-Limited HI Cloud). Our results demonstrate that high-sensitivity
and wide-field HI imaging is important in revealing the diffuse cold gas
structures and tidal debris which is crucial to understanding the dynamical
evolution of galaxies.
