We propose a detailed model for the formation of the lunar crater Hyginus, the associated Hyginus rille, a series of collapse pits along the rille, and what we identify as a blanket of pyroclasts surrounding the crater. We show that the geometry of the rille graben is consistent with its initiation by the intrusion of a dike that did not breach the surface, and that Hyginus crater may be a caldera formed by surface subsidence into a partly evacuated sill that grew from the upper part of the dike. Dike propagation necessarily entails the formation of a gas-filled cavity in the upper tip of the dike, underlain by a layer of magmatic foam. Eruption through the graben boundary faults of a mixture of free gas from the dike tip and vesiculating and fragmenting magmatic foam from the sill provided enough released gas to explain the extent of the pyroclastic deposit. Subsidence of the crust in various places along the graben into the depressurized dike tip gas cavity led to the formation of the collapse pits. The model is strongly supported by the close agreements between, first, the total volume of the pits measured from images and the volume of the dike tip gas cavity predicted by theoretical calculations and, second, the estimated volume of the pyroclastic deposit and the calculated magmatic liquid content of the sill.