Left: low-density regions in a sub-surface layer 200 km below ⟨τ⟩ = 1 sustaining swirls. The colour map extends from blue, indicating low density, to red, indicating high density, and the arrows represent the plasma velocity projected into the horizontal plane. Longest arrows correspond to a velocity of 9.5 km s^-1. Such swirling low density regions are much more abundant than nMBPs but most of them do not extend into the photosphere and therefore do not produce nMBPs. Right: corresponding bolometric intensity map. Of the several low density swirling regions in the left panel only the one at (x,y) = (0,0) produces a nMBP.