Inverted differential opacity distribution in the Galactic plane. The Sun is at coordinates (x,y) = (0,0) and the Galactic center direction to the right. The opacity increases from red to violet (scale to the right). Note the 1000 pc long cavity centered on l ≃  + 225° that we identified as the major super-bubble GSH238+00+09 detected in radio by Heiles (1998). We call Cygnus cavities the empty regions at l ≃  + 70°. Several well known opaque regions are indicated on the maps, and their identification is based on the literature. The gradient axis for interstellar helium ionization points to the center of the GSH238+00+09.