Earth holds an estimated 23.4 million cubic kilometers of groundwater in the upper portion of its crust, making it by far the largest reservoir of liquid freshwater on the planet. That volume is enough to cover the entire land surface of Earth in a layer roughly 180 meters deep. But the full picture is more nuanced than a single number, because how much groundwater exists depends on how deep you count and how much of it is actually usable.
Total Volume and How It Compares
The U.S. Geological Survey estimates that groundwater (both fresh and saline) totals about 23.4 million cubic kilometers. That sounds enormous, but it represents only about 1.7% of all water on Earth, since the oceans hold the vast majority at roughly 1.335 billion cubic kilometers. Where groundwater stands out is in the freshwater category: it accounts for 99% of all liquid freshwater on the planet. Rivers, lakes, and swamps combined hold just the remaining 1%.
To put that in perspective, if you drained every river and lake on Earth, you’d have a tiny fraction of what sits beneath your feet in rock and sediment. Groundwater is also the source of about one quarter of all water humans use globally, supplying drinking water, irrigation, and industry across every continent.
How Deep Does Groundwater Go?
Most estimates of groundwater focus on the upper 2 kilometers of Earth’s crust, where water fills the gaps between grains of rock and sediment in formations called aquifers. A landmark study published in Nature Geoscience, using more than 40,000 groundwater models, estimated that roughly 22.6 million cubic kilometers of groundwater sits within this upper zone.
But water doesn’t stop at 2 kilometers. A University of Arizona study found an additional 20 million cubic kilometers of deep groundwater below that threshold. When you combine shallow and deep groundwater, the total reaches approximately 44 million cubic kilometers. That figure actually surpasses the volume of Earth’s ice sheets, making underground water the single largest reservoir of water on land. This was a surprising finding, since ice sheets had long been considered the dominant land-based water store.
Not All Groundwater Is Usable
The raw volume is misleading if you’re thinking about water supply, because most groundwater is either too deep to access economically or too salty to use without treatment. Deep groundwater, the stuff below 2 kilometers, is largely out of reach with current technology and tends to be highly saline from dissolving minerals over millions of years. Even within the upper 2 kilometers, a significant portion is brackish or saline.
There’s also the question of age. Less than 6% of groundwater in the upper crust is “modern,” meaning it fell as rain or snow within the last 50 years. That modern groundwater, totaling about 0.35 million cubic kilometers, is the portion most connected to the active water cycle. It recharges relatively quickly and tends to be found closer to the surface, making it the most accessible and the most renewable. The other 94% has been underground for centuries, millennia, or even longer. Pumping that ancient water is essentially mining a resource that won’t refill on any human timescale.
Where Groundwater Is Stored
Groundwater doesn’t sit in underground lakes or rivers, despite what many people imagine. It fills microscopic pores and fractures in rock and sediment, somewhat like water saturating a sponge. The amount of water a given area holds depends on the type of rock. Sandstone and gravel can hold enormous quantities because they have large, well-connected pore spaces. Dense granite or clay holds far less.
The water table, the level below which all pore spaces are fully saturated, can be just a few meters underground in wet regions or hundreds of meters deep in arid ones. Major aquifer systems like the Ogallala beneath the U.S. Great Plains, the Great Artesian Basin in Australia, and the Nubian Sandstone Aquifer in North Africa each hold thousands of cubic kilometers of water individually. These systems developed over geological time and in many cases receive very little new recharge compared to how quickly water is being pumped out.
Why the Numbers Keep Changing
Estimates of total groundwater have shifted over the decades as measurement techniques improve. Early figures relied on rough geological models and limited borehole data. More recent studies incorporate satellite gravity measurements, isotope dating of water samples, and large-scale computer modeling to refine the picture. The discovery of vast deep groundwater reserves, for example, only emerged clearly in the mid-2010s. Each new dataset tends to push the total volume estimate higher, particularly for deep and saline water that earlier surveys missed. The accessible freshwater portion, however, has remained relatively stable in estimates, hovering around that 10.6 million cubic kilometer range for shallow fresh groundwater.
What is changing in real time is how much groundwater remains in specific aquifers. Satellite data from NASA’s GRACE mission has shown that many of the world’s largest aquifers are being depleted faster than they recharge. The total volume underground is vast, but the portion that is fresh, accessible, and renewable is a small slice of the whole, and it’s the slice under the most pressure.