  We present 21-cm HI observations from the Southern Galactic Plane Survey of
the field around the anomalous X-ray pulsar 1E 1048.1-5937, a source whose
X-ray properties imply that it is a highly magnetized neutron star (a
"magnetar"). These data reveal an expanding hydrogen shell, GSH 288.3-0.5-28,
centered on 1E 1048.1-5937, with a diameter of 35x23 pc (for a distance of 2.7
kpc) and an expansion velocity of approx 7.5 km/s. We interpret GSH
288.3-0.5-28 as a wind bubble blown by a 30-40 M_sun star, but no such central
star can be readily identified. We suggest that GSH 288.3-0.5-28 is the wind
bubble blown by the massive progenitor of 1E 1048.1-5937, and consequently
propose that magnetars originate from more massive progenitors than do radio
pulsars. This may be evidence that the initial spin period of a neutron star is
correlated with the mass of its progenitor, and implies that the magnetar birth
rate is only a small fraction of that for radio pulsars.
