Mean and standard deviation maps for the initial (top panels) and final density fields (bottom panels), computed from the MCMC realisations, with the same slice through the 3D density fields being illustrated above. In unobserved or masked regions, the density fields are not constrained by data, and they average out to the cosmic mean density. However, in observed regions, the Gaussian nature of the initial conditions and the filamentary nature of the non-linearly evolved density field is manifest. The corresponding variance is therefore higher in regions devoid of data.