DURING its construction a submarine pipeline is at the same time bent and loaded by the external pressure of the sea. If it is bent too far, it buckles. If the local external pressure is small, the buckle is limited to a short kink on the compressed side of the pipe. But if the external pressure is large enough, the local bending buckle can transform itself into a buckle of a different form, which once initiated can propagate along the pipe, driven by external pressure alone, even into regions free of bending. Figure 1 shows a pipe along which a buckle has propagated in this way. The mode can be observed by grasping a tube of toothpaste firmly between thumb and forefinger, and then moving the fingers along the tube. In a short transition region, between the fingers, the pipe bends from a circular into a dumb-bell cross section.