True. But deep contouring can also create some problems for them. Both air and water operate within enclosed reservoirs. The volume of fluid that is displaced by a hip or a shoulder moves to another unpressured part of the bed, usually the corners, and can distort the shape of the sleep surface. It generally produces a 'hammock' or sag effect, exactly what you don't want for natural spine alignment. At the same time, this action can also tighten the surface causing a tough membrane that can actually work against comfort. However, a number of engineering refinements, including isolated and separate chamber reservoirs, and foam toppers, have greatly reduced both the distortion and the membrane feeling in these flotation systems. In foam flotation, the volume of air displaced by the same hip or shoulder moves easily through the open cell structure of the foam and is vented to the atmosphere without distorting the shape of the sleep surface or support system.