Collapsing region at z = 12 and the corresponding halo that forms from that patch of the density field, viewed at z = 0. Only 10% of the particles are plotted for the sake of simplicity. The color code indicates the height on the z-axis at z = 12, from low (purple) to high (yellow). Arrows indicate the initial and final location of a few individual DM particles. Note that this particular region is composed of two disjointed subregions that are in the process of collapsing and merging.