Colour-coded maps of the logarithmic total gas column density in model A for three different times before present. Integrations along the y- and z-axes are shown in the upper and lower panels, respectively. Earth’s projected position is marked by a cross. Circles indicate the projected centres of SN explosions that occurred in the time frame before the snapshot was taken. The sizes of the circles correspond to the initial masses of the progenitor stars (see legend in the upper left panel).