In a hard drive, the heads 'fly' above the disk surface with clearance of as little as 3 nanometres. The "flying height" is constantly decreasing to enable higher areal density. The flying height of the head is controlled by the design of an air-bearing etched onto the disk-facing surface of the "slider". The role of the air bearing is to maintain the flying height constant as the head moves over the surface of the disk. If the head hits the disk's surface, a catastrophic head crash can result.