Opinion of the Court
issued his report, which recommends that this Court dismiss Mississippi’s complaint with leave to amend. Both Mississippi and Tennessee have filed exceptions.
I
A
Layers of rock, clay, silt, sand, and gravel exist below the Earth’s surface. Groundwater percolates through the spaces in and around these materials, sometimes forming underground reservoirs of water known as aquifers. Some aquifers are small, while others span tens of thousands of square miles. The Middle Claiborne Aquifer is one of the latter. It underlies portions of eight States in the Mississippi River Basin: Alabama, Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri, and Tennessee. See Report of Special Master 16; Hearing Tr. 278–279. Many of these States, including Mississippi and Tennessee, draw significant amounts of groundwater from the aquifer. Id., at 660–662, 1038–1040; Joint Exh. J–71.
To extract water from an aquifer, people drill wells. Pumps then draw water to the surface, where it is processed and piped to customers. Pumping does not just bring water to the surface; it also lowers water pressure at the site of the well. Water is naturally drawn to this area of lower pressure. This, in turn, “causes a pattern of lower or depressed water levels around the wells.” Report of Special Master 13. Hydrogeologists call such areas “cones of depression.” These cones of depression can be local—say, the size of a backyard. Or they can be regional, stretching out for many miles from a pumping site. See id., at 21–23; Hearing Tr. 176, 188, 435.
The City of Memphis, through its public utility, the Memphis Light, Gas and Water Division (MLGW), pumps approximately 120 million gallons of groundwater from the Middle Claiborne Aquifer each day. Id., at 186, 200; Pl. Exh. P–157. It does so using more than 160 wells located
