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; 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; wherein the cooling air source is configured to supply cooling air comprising ambient air from outside the data centre and comprising one or more adiabatic cooling units and being free from mechanical cooling units; and wherein the method comprises: controlling the one or more adiabatic cooling units to adjust the humidity and/or temperature of the cooling air; transporting air to be cooled to the cooling air source under the control of the fans of the rack-mountable items of electronic equipment, the air to be cooled comprising air from outside the data centre and optionally air from inside the data centre; and transporting cooling air from the cooling air source to the cold aisles under the control of the fans of the rack-mountable items of electronic equipment. 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; 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; wherein the cooling air source is configured to supply cooling air comprising ambient air from outside the data centre and comprising one or more adiabatic cooling units and being free from mechanical cooling units; and wherein the method comprises: controlling the one or more adiabatic cooling units to adjust the humidity and/or temperature of the cooling air; transporting air to be cooled to the cooling air source under the control of the fans of the rack-mountable items of electronic equipment, the air to be cooled comprising air from outside the data centre and optionally air from inside the data centre; and transporting cooling air from the cooling air source to the cold aisles under the control of the fans of the rack-mountable items of electronic equipment. controlling the one or more adiabatic cooling units to adjust the humidity and/or temperature of the cooling air; transporting air to be cooled to the cooling air source under the control of the fans of the rack-mountable items of electronic equipment, the air to be cooled comprising air from outside the data centre and optionally air from inside the data centre; and transporting cooling air from the cooling air source to the cold aisles under the control of the fans of the rack-mountable items of electronic equipment.