A data center cooling system includes a floor structure defining a below-floor warm-air plenum and an above-floor cool air plenum, a plurality of above-floor computer assemblies arranged to exhaust warmed air into the warm-air plenum, and one or more fan-coil arrangements to draw air from the warm-air plenum, cool the air, and provide the air to the cool air plenum. The volume of the above-floor cool air plenum and the below-floor warm air plenum may both be substantial so as to minimize changes in temperature from the failure of components in the system.