The boundaries between land parcels usually are assumed to be static and unchanging. However, not all land borders are stable. An important land boundary that routinely ambulates is the border between what is publicly and privately owned along U.S. coastal shores. This coastal boundary recently has been the subject of renewed attention from the courts, scholars, and even the popular press in the wake of Hurricane Sandy. This Article offers an economic analysis of why the boundary generally ambulates, rather than remaining perpetually fixed as land borders usually are assumed to do. It also considers whether the legal border generally should continue to migrate in an era of sea level rise due to climate change.