H I density contours from a 25 Mpc h⁻¹ subsection of the overall 100 Mpc h⁻¹ simulation box, labeled by redshifts and their corresponding volume‐weighted mean neutral fractions. The different colors correspond to densities occupying different volume fractions at that redshift. Blue: ﹩f=0.93﹩ or ﹩\nu =-1.5﹩. Green: ﹩f=0.5﹩ or ﹩\nu =0﹩. Red: ﹩f=0.07﹩ or ﹩\nu =1.5﹩. Note that actual threshold density values vary with redshift (see discussion in text).