Three-dimensional snapshot of density at 500 ky of an ionization front propagation with a modulated interface. The UV flux at the top of the box is 10⁹ photons per second per square centimetre, the box is 4 × 2 × 2 parsecs. The dense gas is initially at 500 H/cm³ at a temperature around 25 K (thermal equilibrium from Fig. 1). The (base width)/height ratios (w/h ratios hereafter) of the modulation is w/h = 0.9.