I suspect that's one that got buried by the media.
The Trail Smelter case was back in the 1930's - the first big international environmental law case. It certainly was considered a big deal at the time and had a lot to do with the formulation of air pollution treaties between the U.S. and Canada.
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"Trail, British Columbia was home to the largest lead and zinc smelting complex in the British Empire. Its 400-foot high stacks sent plumes of noxious smoke down the Columbia River valley. Upon crossing the United States - Canadian border, the smoke caused damage to crops and forests in Washington State. American farmers and the Canadian Company locked horns over compensation for crop losses and measures to ensure smoke reduction.
From this relatively unremarkable, private-law beginning, the dispute quickly escalated to become an international conflict involving the highest levels of the United States and Canadian governments. The dispute raised thorny issues of how international law should respond to transboundary air pollution. It took almost 15 years and two precedent-setting decisions from an international arbitration tribunal to finally settle these questions. Today, the Trail Smelter Arbitration is celebrated as the first international ruling on transborder air pollution, having established the "polluter pays" principle as the basis for resolving transboundary environmental disputes and remains one of the key underpinnings of international environmental law."
- from here