In the mountains east of the city, just beyond the curving road up Sir Lowry’s Pass, workers maneuver heavy machinery to stab at the ground near Steenbras Dam, drilling deeply with steel pipes to bring forth water.

They are trying to tap into the Table Mountain Group aquifer, which holds water in a vast lattice of narrow rock fractures beneath the Cape Fold mountains that ring the city. Cape Town has eyed the aquifer as a water source for nearly two decades, but plans were accelerated during a three-year drought in which reservoirs shriveled to the point that Mayor Patricia De Lille threatened to shut off water to homes and businesses, a doomsday scenario branded as Day Zero.

Cape Town, which wants to diversify its water sources beyond reservoirs, is not the only municipality on a subterranean quest. In increasingly dry conditions, cities from Australia and the Middle East to the American Southwest are pursuing groundwater, either as an integral piece of their future water supply or as an emergency stopgap measure.

Los Angeles, looking long-term, aims to double the share of its water supply that comes from groundwater by 2040 and cut reliance on distant and shrinking sources like the Colorado River. Las Vegas, which also uses the Colorado River, has attempted for nearly three decades to acquire rights to groundwater from spring-fed valleys some 300 miles (480 kilometers) north. For Sin City, groundwater rights, which are fiercely contested by environmental groups, ranchers, and valley residents, are a hedge against a declining Lake Mead, currently just above its record low. The big Colorado River reservoir is Las Vegas’s only water source.

In the Australian state of New South Wales, where rainfall deficits over the last year and a half are among the largest on record, reservoirs are also front of mind. Storage plunged by 40 percent in the last 12 months. The inland town of Murrurundi, some 185 miles (300 kilometers) north of Sydney, instituted strict water-use limits in July, and today its main reservoir is below 30 percent of capacity. Once the dam drops below 20 percent, accumulated sediments will make the water increasingly difficult to use, says Philip Hood, manager of water and sewer for the Upper Hunter Shire Council, the regional governing body.

With no rain and at current use, one thousand residents will be out of water in less than 70 days, according to Daele Healy, communications officer for the council.

That is an unacceptable outcome, so Murrurundi is looking to groundwater to supplement the reservoir until a pipeline connecting the town to a larger city’s water system is completed in 2020.

“It won’t take us off restrictions,” Healy told Circle of Blue about the groundwater plan. “But we won’t need to truck in water.”

Increasing Pressure on Groundwater

To satisfy growing water demands, global groundwater use has soared in recent decades — but not all of the growth is sustainable. A 2014 study published in the journal Water Resources Research found that, based on satellite data, the rate of groundwater depletion — meaning more withdrawals than deposits — has doubled since the last decades of the 20th century, largely due to agriculture. The problem spans continents. Most of the world’s largest aquifers registered deficits in the previous decade, according to a 2015 study from the University of California, Irvine that also used satellite data.

That does not mean that any particular site is being managed unsustainably. But it does indicate a worrisome international trend. Evidence is everywhere.

Groundwater depletion in India is so severe that Nitin Gadkari, the water minister, says the country is facing the “worst water crisis” in its history. Much of the deficit is due to rampant farm irrigation — India pumps more groundwater, and more groundwater for farming, than any other country — but some urban centers have so depleted their water tables that earlier this year India’s top environmental court ordered the Delhi government to shut down all illegal wells in the city.

Other megacities — Bangkok, Jakarta, Hanoi, Mexico City, and Tehran for instance — are sinking in areas where too much groundwater has been pumped.

The reason groundwater is so desirable is as true for cities as it is for farmers: groundwater is often the cheapest, most readily available option, especially in a time of immediate need. Despite talk of large facilities to remove salt from the ocean and of reusing sewer water, groundwater is the “best value of money” for Cape Town to bolster its water supply, argues Christine Colvin, a freshwater specialist with WWF-South Africa.

City of Cape Town numbers back this up. Per unit of water, tapping the three local aquifers — the Atlantis, Cape Flats, and Table Mountain Group — is at least two to four times cheaper than water from a desalination plant.

Wastewater recycling and desalination are proven technologies, but are more costly and slower to build. Groundwater generally represents a quicker solution, particularly for individuals and businesses who want to wean themselves from municipal water. Drilling companies working in Cape Town reported being booked for months at the height of the Day Zero panic. City authorities now count more than 26,100 registered household boreholes in the city, mostly in wealthier districts.

Businesses, too, looked underground to outlast the drought. ACA Threads, a manufacturer of sewing thread, plans to switch to groundwater to run its production facility on the outskirts of Cape Town, said Chris Handt, operations manager. The company invested in two boreholes that produce enough water to cover all the facility’s needs. It will remain connected to the municipal system, but only for emergency purposes.

Handt told Circle of Blue that ACA will offset some of its pumped water by redirecting rainwater that normally flows into storm drains into an on-site recharge basin. Handt did not know how effective the recharge plan would be.

Moving Slowly

For Cape Town itself, groundwater was expected to be the quickest means of augmenting its water supply, but that has not been the reality. Fortunately, a heroic water conservation campaign and soaking rains in June helped Cape Town avoid a Day Zero catastrophe. Its reservoirs are now more than 70 percent full. The national government did not renew the drought disaster designation in June, and the city announced it will start relaxing water use restrictions on October 1.

Earlier this year, city officials said they anticipated, in an optimistic scenario, pumping 100 million liters of groundwater a day by October from the three aquifers. They have fallen far short. Twelve million to 15 million liters a day is being pumped, only from the Atlantis aquifer.

Ian Neilson, the executive deputy mayor, lists a number of factors for the slow progress on the Cape Flats aquifer: access to land for drilling, poorer water quality than was expected, and the need to guard the wells against contamination.

The city had to acquire licenses from the national Department of Water and Sanitation to extract groundwater. The license for the Cape Flats aquifer requires the city to replace a portion of the water it pumps out. Recharge is possible because the Cape Flats, near the coast, is a sandy, alluvial aquifer into which water easily filters.

The Table Mountain Group, however, is a fractured rock aquifer, where water flows between innumerable below-ground pathways. City-operated recharge sites are not feasible here.

Understanding these systems — and how pumping will alter ecology and hydrology — requires precise knowledge. Though they are useful for uncovering global and regional trends, satellite data are not sharp enough to reveal groundwater dynamics for particular wells or catchments. For local management, local data about subsurface flows, recharge rates, and water chemistry are needed.

“Emergency groundwater supplies will experience the longer-term effects of the drought, so after the drought the aquifer will need extra time to recover,” Claudia Ruz Vargas, a researcher with the International Groundwater Resources Assessment Center, told Circle of Blue.