States increasingly refer to ‘legal stability’ in connection with maritime zones, amidst concern to preserve their jurisdictional rights in the face of climate-change induced sea-level rise. Yet such a claim for preservation is at odds with the widely-expressed scholarly view that baselines, and their associated maritime zones, ‘ambulate’ with coastal changes. This article interrogates this tension by focussing on the understudied notion of legal stability as it relates to maritime zones, under the international law of the sea. The article examines the development of the term ‘legal stability’ in the discourse of States (what States say) and contends that a claim for the stability of maritime zones should be seen as an expression of the long-standing value placed on legal stability by States in the system of maritime zones. Further, the article presents the results of a global study of States’ implementation of the normal baseline in domestic legislation (what States do). The results show that many States have taken practical measures to secure legal stability for their normal baselines within their domestic frameworks, suggesting that existing international law may accommodate a greater degree of stability than widely appreciated. The article concludes by asserting that these findings matter not only for how we should receive States’ claims for maritime zone preservation on the basis of legal stability, but also prompts reconsideration of our overall understanding of the existing law on baselines and maritime zones.