A cool (ha ha!) solution was the old Cray XT3/4 supercomputers, which were air cooled. But instead of a battalion of tiny fans, each cabinet had a single huge f

A cool (ha ha!) solution was the old Cray XT3/4 supercomputers, which were air cooled. But instead of a battalion of tiny fans, each cabinet had a single huge fan at the bottom, blowing air vertically through the cabinet (the boards were mounted vertically). No redundancy, sure, but AFAIU it was reliable enough to not be a problem in practice.