Histories of American water have tended to flow in two dimensions, tracing surface waters across horizontal space and only rarely descending into the earth below. Rivers, wetlands, dams, and canals have defined how we understand our relationship with this vital resource, and with the diverse human and more-than-human actors who share it. Foundational works by Donald Worster and Norris Hundley traced the rise of hydraulic societies in the American West, showing how rivers, dams, and aqueducts enabled both state formation and capitalist expansion, while Joel Tarr and Martin Melosi foregrounded water infrastructure in the field of urban environmental history.Footnote ¹ More recent scholarship has explored cultural and social histories of water, including more-than-human perspectives, environmental justice, Indigenous water rights, and the political ecologies of urban and rural water systems, expanding the field well beyond its earlier focus on engineering and state-building.Footnote ²

But other stories lie just below the surface.Footnote ³ This essay calls for a three-dimensional approach to American water history. Water flows, land is permeable, and surface waters are physically and politically inextricable from the subterranean waterscapes beneath them. For over a century, large-scale groundwater use has supported settlement and population growth across the arid regions of the world, sustaining the cities, farms, and industries that make modern life possible. It has allowed settler states to tighten their hold on dispossessed land by extending imperial authority into subsurface space, deepening inequalities of access and use, and helping secure territorial and economic control. Today, groundwater provides nearly half of all drinking water worldwide and supports approximately 40 percent of global irrigation. In the United States, this reliance translates into water for about half the population and 27 percent of irrigated land. Dependence on groundwater is expected to increase in the coming years due to climate change, as surface water becomes scarcer and its availability more erratic.Footnote ⁴

While deep aquifers are somewhat buffered from short-term climatic changes, they are far from immune to broader changes in the global system. Patterns of unsustainable use established over the last century have led to widespread overdraft, raising pumping costs and threatening the long-term viability of vital agricultural and urban regions around the world. Falling water tables and declining hydrostatic pressure have reversed rivers, desiccated ancient springs and marshes, and caused the catastrophic subsidence of the land itself. Climate-induced reductions in surface water availability will only intensify these dynamics by increasing demands on groundwater while reducing recharge, the natural refilling of groundwater stores through rainfall or stream seepage. This feedback loop accelerates depletion and hastens the point at which pumping from the depths becomes economically unviable. The consequences threaten food production across some of the world’s most vital agricultural regions, from northern China and the Indo-Gangetic Plain to the Ogallala Aquifer and California’s Central Valley.

For all this, the planetary impact of groundwater use remains strikingly underrecognized in historical accounts of the Anthropocene. The very few historical works focusing directly on groundwater depletion have dealt exclusively with the Ogallala Aquifer, where scholars have explored themes of agricultural intensification, rural decline, and more recently environmental justice.Footnote ⁵ In other geographical contexts, a few historians have addressed aquifer contamination as an issue of public health, while others have acknowledged groundwater as one component of a broader hydraulic system.Footnote ⁶ Most accounts of historical groundwater development appear in works of geology or public policy, where it is treated primarily as a technical or regulatory concern rather than as part of historical processes of social and environmental change.Footnote ⁷ This is a significant omission, as the Ogallala is far from the only aquifer to shape and be shaped by social, economic, political, and ecological systems.

California’s Central Valley, far better known for the courtroom battles, local feuds, and reclamation projects that have characterized residents’ long use of surface water, likewise rests on and has been transformed by its hidden groundwater. This is perhaps the best-studied waterscape in the world, attracting scholarship across the humanities and natural sciences for its economic, legal, and environmental significance, yet to date no historical study has fully acknowledged the central role of subsurface water.Footnote ⁸ Prior to large-scale development in the mid-nineteenth century, the surface of the valley sat atop a network of interconnected shallow aquifers with a relatively high water table, which stored water even when surface flows evaporated. Seepage from these aquifers contributed over half of the total volume of water in the watercourses and wetlands above and sustained them during dry seasons when precipitation and runoff were scarce. Deeper down, hundreds of feet below the surface, lay far larger aquifers, confined by a thick, low-permeability clay layer called an aquitard, which recharged more slowly from water filtering through alluvial sediments from intake zones along the Sierra Nevada foothills.Footnote ⁹

For millennia, Indigenous communities in the valley engaged with springs, seeps, and groundwater-fed wetlands as part of seasonal cycles and land-based knowledge systems. Grounded in subsistence, ceremony, and ecological knowledge, these relationships to water were disrupted and largely displaced by changes wrought by ranchers, prospectors, and farmers in the nineteenth century. The new arrivals first dug wells that reached only the shallow aquifers, using windmills and buckets to draw water for kitchen gardens and personal use. But by the 1880s, steam-powered drilling rigs opened access to the deeper aquifers, and hydrostatic pressure brought millions of gallons of water bubbling to the surface. Railroad companies and land speculators used these artesian flows to promote the sale of non-riparian lands, luring settlers with the promise of abundant, freely flowing water. As the pressure inevitably fell and the flows failed, these recent arrivals were among the hardest hit. The resulting land abandonments and forced sales accelerated the consolidation of resources and power among the Valley’s wealthiest landowners.Footnote ¹⁰

That agrarian elite turned to turbine pumps to raise the water the remaining distance to the surface and divert it into private canals, effectively transforming subsurface flows into surface water they could control and sell.Footnote ¹¹ While not commented upon in the literature, this practice collapsed the distinction between groundwater and surface supplies, implicating both in the ongoing contests over irrigation rights and costs. Because pumping almost immediately exceeded natural recharge, the water table dropped, forcing pumps to work harder and making the water they produced increasingly expensive. For smallholders and immigrant farmers, the rising costs proved ruinous, accelerating their displacement and further concentrating land and power.Footnote ¹²

By the 1940s, groundwater pumping in the valley had reversed natural pressure gradients, severing the groundwater–surface water connections and drawing water downward from the shallow aquifers. The baseflow that had sustained dry-season streamflow across much of the valley diminished, then disappeared, significantly reducing the availability of surface water. To compensate, irrigation districts and large landowners leaned more heavily on deep-well pumping, intensifying overdraft and accelerating the collapse of the hydrological system as a whole.Footnote ¹³

To alleviate pressure on the collapsing aquifers, the state and federal government sought to provide additional surface water via the contentious and costly Central Valley Project (CVP). As with so many reclamation projects of the time, the CVP did not have its intended effect: groundwater pumping continued to increase, and the new dams compounded the problem by capturing many of the eastern streams that had once recharged the shallow aquifers. The subsequent reductions of baseflow pushed irrigators toward still greater reliance on deep pumping, and later imports from the State Water Project likewise failed to alleviate the problem. By the 1960s, groundwater had surpassed surface water as the dominant source of irrigation in many parts of the valley, and land subsidence in the areas of the most intense pumping (caused by the loss of hydrostatic pressure) damaged infrastructure and created heightened flood risks.Footnote ¹⁴

Water users in the Valley, politicians in Sacramento, and hydrogeologists across the country clearly understood the relationships between surface and groundwater, but California’s legal frameworks continued to treat them as separate and independent resources. While litigation, legislation, regulation, and construction dealing with surface water abounded, efforts to regulate groundwater use were few and far between. The only substantive effort at reform, a bill introduced in 1978 that would have required major water users to develop management plans for overexploited groundwater basins, faced such vehement opposition from the Central Valley’s irrigation lobby that its sponsor was forced to amend it down to a call for additional research on the state’s hydrogeology. Centralized, enforceable management authority for California’s groundwater would be enacted only in 2014, when the aquifers were already critically and, in some cases, irreparably overdrawn.Footnote ¹⁵

Compared to the well-documented conflicts over surface water, the long history of groundwater extraction in the Central Valley has remained largely unexamined. This reflects more than simple physical invisibility, as the Valley’s powerful landowners and irrigation districts had every incentive to keep groundwater off the political agenda and their own practices out of public view. Debates over groundwater regulation took place behind closed doors, not in the newspapers, and the few substantive efforts to implement legislative reforms were killed in committee or lost in protracted lawsuits. They have accordingly left only faint traces in the archive, buried in the papers of a bill’s sponsor or referred to obliquely in references to the possibility of “underground storage” of water from the CVP.Footnote ¹⁶

Attending to these faint traces and rereading the Central Valley and other landscapes with attention to depth can reveal new dimensions of water’s entanglement with power. Inequalities developed vertically as well as horizontally, between those with the capital for deep wells and powerful pumps and those whose access to water was confined to the surface. This dynamic continues today: agribusinesses overdraw the aquifers to irrigate the almond orchards and vineyards of a multi-billion-dollar industry, while the wells of unincorporated rural towns—overwhelmingly Latino farmworker settlements—and Native American communities run dry.Footnote ¹⁷ The politics of groundwater expose how unequal power over depth continues to shape water justice in California.

This case demonstrates why environmental history must take subterranean as well as surface waters seriously, advancing the call for three-dimensional histories posed at the outset. Groundwater’s physical, legal, and archival obscurity in California and beyond has allowed its depletion to proceed largely out of sight; bringing it into historical focus can reveal the central role it has played in the development of modern states and economies. Attending to these hidden resources reshapes our understanding not only of the Central Valley, but of the Anthropocene more broadly. Key agricultural regions worldwide, from California and the Arizona basin to the Argentine Pampas and Australia’s Murray-Darling Basin, depend on the conjunctive use of surface and subsurface water. Each is shaped by its own complex history of human and nonhuman entanglements. Turning our gaze downward is essential to grasp the full complexity of these arid and semiarid worlds.