  Stephan's Quintet (SQ, distance=85$\pm$6 Mpc) is unique among compact groups
of galaxies. Observations have previously shown that interactions between
multiple members, including a high-speed intruder galaxy currently colliding
into the intragroup medium, have likely generated tidal debris in the form of
multiple gaseous and stellar filaments, the formation of tidal dwarfs and
intragroup-medium starbursts, as well as widespread intergalactic shocked gas.
The details and timing of the interactions/collisions remain poorly understood
because of the multiple nature. Here we report atomic hydrogen (HI)
observations in the vicinity of SQ with a smoothed sensitivity of 1$\sigma$=4.2
$\times 10^{16}\rm cm^{-2}$ per channel ($\Delta$v=20 km s$^{-1}$;
angular-resolution=4'), which are about two orders of magnitude deeper than
previous observations. The data reveal a large HI structure (linear scale ~0.6
Mpc) encompassing an extended source of size ~0.4 Mpc associated with the
debris field and a curved diffuse feature of length ~0.5 Mpc attached to the
south edge of the extended source. The diffuse feature was likely produced by
tidal interactions in early stages of SQ (>1 Gyr ago), though it is not clear
how the low density HI gas (N$_{\rm HI}\leq 10^{18}\rm cm^{-2}$) can survive
the ionization by the inter-galactic UV background on such a long time scale.
Our observations require a rethinking of gas in outer parts of galaxy groups
and demand complex modeling of different phases of the intragroup medium in
simulations of group formation.
