  We produce a detailed map of the perturbed surface density of neutral
hydrogen in the outer Milky Way disk demonstrating that the Galaxy is a
non-axisymmetric multi-armed spiral. Spiral structure in the southern half of
the Galaxy can be traced out to at least 25 kpc, implying a minimum radius for
the gas disk. Overdensities in the surface density are coincident with regions
of reduced gas thickness. The ratio of the surface density to the local median
surface density is relatively constant along an arm. Logarithmic spirals can be
fit to the arms with pitch angles of 20-25 degrees.
