INTERSTATE GROUNDWATER LAW REVISITED: MISSISSIPPI V. TENNESSEE Noah D. Hall* Joseph Regalia** In June 2015, the United States Supreme Court granted the State of Mississippi leave to file a bill of complaint against the State of Tennessee, the City of Memphis, and Memphis Light, Gas & Water Division for wrongfully converting groundwater from the interstate Sparta-Memphis Aquifer. The dispute arises from Memphis and its municipal utility pumping groundwater within Tennessee, which Mississippi alleges has lowered the water tables within its territory. The Supreme Court's grant of leave raises for the first time the question of what legal doctrine applies to transboundary interstate groundwater resources. Tennessee and lower courts would subject interstate groundwater to the Court's equitable apportionment doctrine, which divides and allocates interstate surface waters by determining the best overall utility for the water supply with a heavy emphasis on protecting existing consumptive uses. Mississippi's bill of complaint seeks damages and declaratory relief based on property theories of absolute right, title, and exclusive possessory ownership of groundwater located within its territorial borders. This article offers a third alternative, the Supreme Court's doctrine of interstate nuisance, which recognizes and balances competing sovereign interests in utilization and preservation of shared interstate natural resources