This paper deals with the possible disappearance of some island States as a result of one of the negative effects of climate change, i.e., the sea level rise. In this sense, this phenomenon of ‘disappearing island States’ implies serious legal challenges to the Law of the Sea, the International Law and their alleged universality. Among others, the impact of the sea level rise on the maritime boundaries of the disappearing island States in addition to the feasibility of the proposed options to physically preserve the territory of the disappearing States are considered. From a Public International Law perspective, concerns as the possible disappearance of the constitutive statehood prerequisites (mainly, population and territory, which would progressively become uninhabitable), the need to avoid the statelessness of the disappearing island States’ nationals, or the hypothetical relocation of those ‘climate change refugees’ are analysed, too. Moreover, this contribution addresses the different strategies proposed in order to preserve the international legal personality of those States, namely: the acquisition of territory from other State, the federation with other States, the creation of governments in exile or the self-governing alternatives. In this sense, the studied alternatives seem unlikely to keep recognized over long time. Finally, consideration is given to the influence of this phenomenon in the universality of International Law, taking into account the principles and aims included in the Law of the Sea Convention (and its Preamble). The challenge of this phenomenon to the universality of International Law is based on the tough paradox according to which the smallest contributors to climate change (Small Island States) will be the most affected by climate change - even disappearing - and meanwhile their losses could eventually benefit some of the biggest contributors to climate change.