Abstract: An underdrain for use with granular type liquid filtering systems including a plate of an inverted dished configuration having a plurality of laterally facing slots defined therein, the slots being defined by an upper edge and an associated lower edge, the upper edge being positioned almost directly above the lower edge such that the slots face horizontally outward, the slots being positioned upon the dished plate in a spiral array with each slot being formed by shearing of the plate material between two cutting edges positioned above and below the plate, the upper edges presenting downwardly facing arcuate edges and the lower edges presenting upwardly facing arcuate edges to define a slot having opposed facing arcuate sides, the plate may further define a flat horizontally extending imperforate annular center-section within the spiral array of slots and a flat horizontally extending imperforate annular outer-section extending about the spiral array of slots, the center-section may define a central aperture t