The 1997 decision of the International Court of Justice (ICJ) in the Gabcikovo-Nagymaros case reshaped international law on transboundary resources in the disguise of adhering to customary principles. Aside from praise for the efficient norms it prescribed, the decision raises a fundamental puzzle: from where did the ICJ draw its authority to rewrite international law? Clearly, the ICJ paid no attention to the traditional sources of international law – general and consistent state practice coupled with opinio juris, namely the “belief that this practice is rendered obligatory” – to trace the evolution of customary law. Such faithful inspection could not have led the ICJ to the same conclusions. In September 1997, when the Gabcikovo-Nagymaros decision was rendered, state practice and opinio juris were rather precarious stilts to serve as the foundation of modern transboundary resources law. Instead of compiling, inspecting and analyzing state practice, the ICJ took a short cut by invoking the 1997 Convention on the Law of the Non-Navigational Uses of International Watercourses (“the Watercourses Convention”) under the unsupported assertion that the Watercourses Convention reflected contemporary customary law. This was a bold move: the Watercourses Convention had been adopted less than four months earlier, had no signatories at the time, and its entry into force was far off.