  We report depolarization of extragalactic sources in the NRAO VLA Sky Survey
(NVSS) by local structures in the interstellar medium. The sky density of
polarized sources drops by a factor 2 - 4 in regions with angular scales
approx. 10 degrees, implying up to 40% depolarization on average per source.
Some of these polarization shadows are associated with HII regions, but three
are associated with regions of depolarized diffuse Galactic emission. The
absence of a correlation between the depth of polarization shadows and H-alpha
intensity suggests that some shadows are related to structure in the magnetic
field. At least some polarization shadows are caused by partial bandwidth
depolarization in the NVSS. Alternatively, some may be caused by regions with
small scale (< 1 arcsec) variations in rotation measure.
