A system for storing data includes a rack and one or more data storage modules coupled to the rack. The data storage modules may include a chassis, two or more backplanes coupled to the chassis, one or more mass storage devices coupled to the backplanes, and one or more air plenums extending beneath one or more of the backplanes in the chassis interior. The backplanes are coupled to the chassis in a staggered configuration, where one of the backplanes is coupled to the chassis at a different elevation relative to another one of the backplanes coupled to the chassis. An air passage may extend beneath one or more of the staggered backplanes and may progressively narrow from the inlet end towards the exhaust end of the chassis. A progressively narrowing air passage can progressively impede airflow through the air passage from the inlet end towards the exhaust end.