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In Mississippi v. Tennessee, a case currently on the U.S. Supreme Court’s docket, Mississippi claims that it owns all groundwater stored underneath its borders that does not cross into Tennessee under “natural predevelopment” conditions—before the advent of modern well technology. Mississippi seeks more than six hundred million dollars for pumping by Tennessee wells that tap into a geologic formation that underlies both states. This is a remarkable claim that departs from the almost uniformly established proposition that the states do not “own” the water within their borders, but instead are authorized to manage that water for the “use” of their citizens. It also departs from past recognition of “equitable apportionment” as the doctrine under which the Supreme Court has resolved interstate surface water conflicts, determining relative rights of use rather than awarding monetary damages. This Article situates the conflict at the crossroads of two broader issues. First, under a phenomenon this Article dubs “groundwater exceptionalism,” often the law treats groundwater differently than surface water, partly as a relic of slow-developing hydrologic knowledge. Second, the dispute goes to the very heart of property law and the meaning of ownership, as distinguished from rights of use. The lower courts have consistently framed this decade-long dispute as a matter of competing uses, but have also interjected the rhetoric of ownership into their opinions. This conflation of use and ownership has the potential to affect the outcome of this case, as well as distort future litigation involving equitable apportionment, regulatory takings, state water rights law, and other legal doctrines.

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