A method of cooling a data centre , the data centre comprising: a cooling air source ; at least one rack room having a floor and a plurality of rack storage areas on the floor, each rack storage area accommodating a plurality of racks in which a plurality of rack-mountable items of electronic equipment having at least one fan are housed; multiple cold aisles in the rack room, each cold aisle being adjacent to a rack storage area; multiple hot aisles in the rack room, each hot aisle being adjacent to a rack storage area; and, an air supply corridor for transporting cooling air , above the floor, from the cooling air source to the cold aisles , the air supply corridor having a cross-sectional area of at least 3 m 2 along 90% of its length; characterised in that the cooling air source is an indirect air handling unit having an external air flow path segregated from an internal air flow path and comprising one or more heat exchangers for exchanging heat between air flowing along the internal air flow path and air flowing along the external air flow path and, wherein the internal air flow path of the indirect air handling unit is in fluid communication with the cold aisles of the data centre , and in that the method comprises: transporting cooling air from the cooling air source to the cold aisles and back to the cooling air source under the control of the fans of the rack-mountable items of electronic equipment , wherein the cooling air flows around the data centre in a closed loop extending from the indirect air handling unit to the electronic equipment and back to the indirect air handling unit. a cooling air source ; at least one rack room having a floor and a plurality of rack storage areas on the floor, each rack storage area accommodating a plurality of racks in which a plurality of rack-mountable items of electronic equipment having at least one fan are housed; multiple cold aisles in the rack room, each cold aisle being adjacent to a rack storage area; multiple hot aisles in the rack room, each hot aisle being adjacent to a rack storage area; and, an air supply corridor for transporting cooling air , above the floor, from the cooling air source to the cold aisles , the air supply corridor having a cross-sectional area of at least 3 m 2 along 90% of its length; characterised in that the cooling air source is an indirect air handling unit having an external air flow path segregated from an internal air flow path and comprising one or more heat exchangers for exchanging heat between air flowing along the internal air flow path and air flowing along the external air flow path and, wherein the internal air flow path of the indirect air handling unit is in fluid communication with the cold aisles of the data centre , and in that the method comprises: transporting cooling air from the cooling air source to the cold aisles and back to the cooling air source under the control of the fans of the rack-mountable items of electronic equipment , wherein the cooling air flows around the data centre in a closed loop extending from the indirect air handling unit to the electronic equipment and back to the indirect air handling unit. transporting cooling air from the cooling air source to the cold aisles and back to the cooling air source under the control of the fans of the rack-mountable items of electronic equipment , wherein the cooling air flows around the data centre in a closed loop extending from the indirect air handling unit to the electronic equipment and back to the indirect air handling unit. <EOS>