The rate of innovation in IT system design and especially in High Performance Computing continues to be very high. To keep pace TU Dresden has constructed its new data center using the Plenum concept. The traditional raised floor was substituted by a full building story, creating a highly flexible space to transport power, water, and air. A strict hot-aisle air separation is used and the computer room air-handling (CRAH) units in downflow configuration are positioned directly beneath the hot aisles. This unique arrangement necessitates an unconventional downward flow of hot air from the enclosed hot aisle. Extensive testing has been performed in a cluster of 24 racks (12 per side) equipped with (3+1)×100 kW CRAH unit cooling capacity and 60 test fixtures (air heaters) with 5-15 kW heating power each. Our analysis demonstrates the extremely high efficiency of this air cooling concept even in high-density configurations, up to at least 30 kW per rack. This efficiency is mostly due to the very short airflow paths and wide open cross-sections. We also showcase that no malicious thermal stratification occurs in our hot air downflow configuration. A detailed analysis of the CRAH controls for temperature (through cooling water flow modulation) and airflow (fan speed) highlights the challenges of such control systems in enclosed hot aisle configurations at high power density and short feedback loops. The analysis also considers dynamically changing load patterns including very low partial load scenarios and aspects of operational reliability.