Large-scale Hα environmentof the CMa shell from the SHASSA survey. The faint emission is enhanced down to a surface brightness of ~150 dR. Conspicuous Hα filaments are visible, which are well correlated with the radio map of Gaylard & Kemball (1984) (their Fig. 1), in particular, with features that we outline in green in this figure, called Northern Arc and Southern Loop (which is well correlated with the southern half of the CMa shell). We suggest that this faint Hα emission is associated with a superbubble ~140 pc in diameter (here outlined in the form of the dashed cyan ellipse), analogously to Barnard’s Loop around the Orion nebula.