Lake Constance (Bodensee), situated at the juncture of Germany, Austria, and Switzerland, constitutes an exceptional case in European border studies: a major transboundary lake lacking a clearly demarcated international frontier. Unlike other shared lakes such as Geneva or Peipus, it has long operated as a de facto tripartite condominium—Switzerland advocating a median-line division, while Germany and Austria favor shared sovereignty. Despite the absence of a formal settlement, no state has attempted to impose its claim by force, rendering Lake Constance a paradigmatic example of a “border dispute without conflict.” The durability of this arrangement reflects deeper historical traditions of cooperative governance. Since the Middle Ages, fishing communities established guild-based regulations—seasonal bans, catch limits, and gear restrictions—that anticipated what Elinor Ostrom would later theorize as effective commons management. These measures safeguarded ecological balance and sustained livelihoods, precluding the “tragedy of the commons.” Lake Constance exemplifies what Michel Foucault described as a heterotopia: a liminal space where overlapping sovereignties coexist without rigid partition. Its legal indeterminacy has encouraged cross-border flows and joint environmental stewardship while containing latent tensions. As such, Lake Constance unsettles conventional assumptions of fixed territorial boundaries, illustrating how negotiated and flexible regimes can transform contested frontiers into durable frameworks of cooperation and peace.