Unsteady, perhaps quasi-periodic, behaviour (Meier and Post 1987) of a tidewater glacier that undergoes alternating episodes of slow advance and rapid retreat. The conditions permitting advance to the advanced position in the first place, and the triggers for subsequent unstable retreat, are both poorly understood, although they may involve variations in basal water pressure and probably involve variations of the climatic mass balance. Once retreat has begun, however, observation and simulation (Schoof 2007) agree that, if the bed is grounded below sea level but has a slope opposed to that of the surface, the retreat will continue until the grounding line reaches a part of the bed that slopes in the same direction as the surface. During this unstable retreat, enhanced calving leads to a positive feedback in which accelerated flow and dynamic thinning extend far upglacier from the part that is grounded below sea level. Mass loss is far greater than, and essentially independent of, the climatic mass balance.