  The under-abundance of very massive galaxies in the universe is frequently
attributed to the effect of galactic winds. Although ionized galactic winds are
readily observable most of the expelled mass is likely in cooler atomic and
molecular phases. Expanding molecular shells observed in starburst systems such
as NGC 253 and M 82 may facilitate the entrainment of molecular gas in the
wind. While shell properties are well constrained, determining the amount of
outflowing gas emerging from such shells and the connection between this gas
and the ionized wind requires spatial resolution <100 pc coupled with
sensitivity to a wide range of spatial scales, hitherto not available. Here we
report observations of NGC 253, a nearby starburst galaxy (D~3.4 Mpc) known to
possess a wind, which trace the cool molecular wind at 50 pc resolution. At
this resolution the extraplanar molecular gas closely tracks the H{\alpha}
filaments, and it appears connected to molecular expanding shells located in
the starburst region. These observations allow us to directly measure the
molecular outflow rate to be > 3 Msun/yr and likely ~9 Msun/yr. This implies a
ratio of mass-outflow rate to star formation rate of at least {\eta}~1-3,
establishing the importance of the starburst-driven wind in limiting the star
formation activity and the final stellar content.
