Vertical sections through the nMBP shown in Fig. 1. The background colour shows mass density, where the brightness encodes the sign of the velocity perpendicular to the plane of projection: bright is plasma flowing out of the plane and dark is plasma flowing into it. The red curve indicates the spine of the nMBP, a “valley” in density, found as a sequence of local density minima in horizontal planes. The blue curve indicates the Rosseland optical depth unity. The legend gives intensity, density, and gas pressure contrast, firstly with respect to the neighbourhood and secondly with respect to the global average (at τ = 1 for the density and the pressure).