The streets are flooding in Jakarta, Indonesia. But people just go about their lives. Adults wade through water to work, shop and care for their families. Children splash and play in water-logged streets. This flooding isn’t caused by a storm. It’s not even raining. Flooding here happens every day, as high tides swamp the city’s low-lying areas.

Part of the problem is rising sea levels. But Jakarta has a bigger issue: Its land is literally sinking.

This sinking — what scientists call subsidence (Sub-SY-dunce) — has slowed over the past two decades. At its peak, Jakarta sank more than 280 millimeters (11 inches) per year! Now it’s just around 30 millimeters (about 1 inch) per year. Sinking and higher tides cause seawater to flood about one-fourth of the city. If things don’t change, more than one-third of Jakarta could be underwater in another 25 years.

And Jakarta isn’t alone. Cities are sinking across the globe. Although some subsidence happens naturally, human activities drive most of it.

The good news: People are finding ways to slow and even reverse that sinking.

As land returns to higher levels, flooding in many of these places will stop. Wells will continue to provide water instead of running dry. And innovative ways of fighting subsidence are helping to return nature to cities and surrounding areas.

People — a growing problem

Although earthquakes can cause land to abruptly sink, such natural events aren’t the main cause of subsidence. People are. Or rather, our need for underground resources, especially water.

Most fresh water on Earth is locked up as ice. Just one percent of the world’s fresh water exists in rivers and lakes. The rest of that liquid water lies beneath the surface. It’s known as groundwater.

In many parts of the world, people drill down into aquifers to pump that water out. It’s an important source of water for millions of people. But in some places, that pumping is causing the land to sink.

It’s a particularly big problem for large cities. The more people who live there, the more water they’ll need to pump from the ground.

Not all cities sink equally fast. Geology plays a role, too, notes Leonard Ohenhen. He works at the University of California, Irvine. As a geodesist (Gee-AH-duh-sizt), he monitors and measures the Earth. “Some cities sit on solid, hard rock that barely compresses,” he says. “Others are built on thick layers of clay, sand or sediment that can compress easily.”

Cities atop sediment are most likely to sink.

Ohenhen’s team showed subsidence is a growing issue for cities across the United States. They looked at the 28 most populated cities, many of which are inland. Their estimates suggest that at least 20 percent of the urban area is sinking in each of those cities, mainly due to groundwater pumping. In all, some 34 million people are affected. The researchers shared their results May 8 in Nature Cities.

Subsidence comes with real, but often invisible, problems for the built objects that support a city. Think highways, railways and bridges. These “can crack or break when one section sinks more than another,” Ohenhen says. Water and sewer lines can break as the land beneath them shifts. And buildings, when not constructed properly, can begin to break apart. Ohenhen’s team found that in the U.S. cities it surveyed, more than 29,000 buildings are located in areas facing a high or very high risk of damage.

New construction can boost subsidence, says Pei-Chin Wu. She’s a geologist at the University of Rhode Island in Kingston. To improve safety during construction in areas with soft soil, workers pump out water before adding the foundation for a new building.

Afterward, she says, they don’t usually replace the water they’d taken out. So the area subsides.

Wu grew up in Taiwan’s capital city, Taipei. It sinks and rises through the seasons. The whole city subsides during the dry season, as people pump out groundwater. “When the rains come — and when the water level goes up — it will recover,” she says.

The rains filter down into deep aquifers under the city. This pushes the land up again. The process doesn’t happen everywhere, though. In places where there’s been lots of construction, the ground never fully recovers.

Wu uses satellite data to study subsidence in coastal cities around the world. Those satellites bounce a signal off the land. Computers calculate the distance to the ground based on how long it takes for each signal to return. This shows the height of the land. By comparing repeated measurements for the same site, geologists can detect small changes in height over time.

In a 2022 study, Wu found that subsidence is a bigger problem, globally, than people had realized. And it’s been happening fastest in southeast Asia. Why? Cities there are growing quickly.

Subsidence is slowing in many cities, too, she found. It’s still ongoing, but the rate of fall is not as fast as it was 10 years ago. That’s thanks to the removal of less groundwater.

But not every city is slowing. In some cities — and smaller areas within many cities — the land continues to drop. This makes these areas more likely to flood.

Returning water to the ground

In mainland China, flooding has become a big problem. So has drought. Northern cities often get little rainfall. Those in the south can get way too much. So China developed a sponge city plan to balance the two.

Sponge cities use green roofs (covered by grass or other plants), permeable pavements and other features to get water off of streets as quickly as possible, explains Michele Lancia. He works at the Eastern Institute of Technology, Ningbo, in China. As a hydrogeologist, he studies how water interacts with rock and soil.

In south China, these features protect people from flooding. In the north, collected water can be “stored, purified and reused,” he says. The goal is the “collection and reuse of 65 to 100 percent of the urban rainfall throughout China.”

Managing water use — and reuse — could also help prevent future subsidence. The key is getting water into aquifers. This process is known as recharging. But China’s current system isn’t designed to do that.

Lancia is exploring new ways to recharge aquifers. Where successful, it could reduce flooding while ensuring water is available. “The sponge city concept will be more and more effective in the future,” he predicts. It should save lives as the climate continues to change.

China isn’t the only place where flooding and water supply are an issue. In Virginia, in the United States, the Hampton Roads Sanitation District (HRSD) hopes to address both problems — and reverse subsidence along the way. They developed the Sustainable Water Initiative for Tomorrow, or SWIFT, program. SWIFT aims to return wastewater to the ground. First, the water must be purified until it’s clean enough to drink. Then HRSD pumps it deep underground into what’s known as the Potomac Aquifer.

“Back in [the] early 1900s, you’d put a well in the Potomac Aquifer,” says HRSD hydrologist Dan Holloway. “Water would come out of the well above your head. You wouldn’t need a pump. You’d need a valve” to shut it off.  

That water pressure dropped over the past century as people removed more and more water from the aquifer. Wells slowed from gushes to drips — then to nothing at all. People finally had to start pumping. The level of the water in the aquifer has since dropped about 38 meters (125 feet).

Pumping created another problem: salty groundwater. Fresh water naturally flows downhill from the rolling hills of central Virginia out toward the sea. That fresh water pushes out toward saltwater, Holloway explains. In the past, that flow has kept the groundwater largely salt-free.

Holloway likens it to a bed sheet pulled tight at a sloping angle. A marble on the sheet “will always roll downhill.” But a groundwater pump makes a hole in the water level around the pump. The more that is pumped, the bigger the hole. Removing too much water in that spot is like pinching that bed sheet and pulling it down. “Now both directions want to come in,” he says. “The marble rolls towards the hole.” Fresh water continues to flow from farther inland. At the same time, salty seawater now moves in from the coast. This can make the groundwater salty.

By adding purified wastewater to the aquifer, SWIFT is designed to combat both subsidence and the intruding saltwater. And it works.

“When we put water in the ground, the aquifer expands,” Holloway says. The ground level nearby rises in small but measurable amounts. The program is still in early stages, returning 3.8 million liters (1 million gallons) to the ground each day.

The goal over the next five years is to increase water additions 50-fold. Eventually, SWIFT could stop land subsidence in the area and ensure a continuous source of fresh drinking water for millions of people.

Putting nature to work

In Europe, the Netherlands is known for dikes that hold back the sea. “Almost half of the Netherlands is below sea level,” notes Tom Wils. He works in the country at Utrecht University. As a physical geographer, he studies landscapes. Without dikes, this nation’s low-lying peatlands would be underwater.

These areas weren’t always below sea level. Around the year 900, the peatland “was about 2 meters [6.5 feet] above sea level,” Wils says. Peat forms in wet spots, where moss and other plants partially decay. A carbon-rich layer builds up over time that can be used as fertilizer or fuel. But the land was too soggy to make good farmland. So the Dutch began to drain these bogs. That’s when their land subsidence started, Wils says. 

Today, the Dutch use their peatlands to raise dairy cattle. But they must first drain off excess water. Otherwise, this land is too wet to support the cows. So the land continues to sink.

The Dutch peatlands now lie 2 meters (6.5 feet) below sea level — a drop of 4 meters (13 feet) in most areas. The drop goes even deeper in places where people have harvested peat for fuel.

Wils is studying new ways to reduce subsidence while supporting the land and people here.

One approach is called paludiculture (Puh-LOO-duh-kul-tchur). That’s a fancy term for “cultivating crops that grow in wet environments,” Wils explains. These include cattails and reeds — two types of fibrous plants that can be used as construction materials. Cranberries and rice can be grown at such sites for food.

Adding water back to the land fluffs up the peat. It also allows new peat to form, which raises the height of the land.

Most Dutch paludiculture fields are experimental. But the country now hosts one successful cranberry farm.

Of course, there are challenges to this wet farming. Flooded fields can release methane (CH₄), a powerful greenhouse gas. It’s a stronger threat to climate change than is carbon dioxide (CO₂). Flooded rice paddies commonly spew methane. So Wils is studying ways to cut their release of this gas. He doesn’t want a fix for subsidence that worsens climate change.

Nature can help with subsidence, too. Because peatlands are often part of river deltas, they can be crisscrossed by many streams and rivers. When they flood, nearby reeds, trees and other plants can trap sediments to help form new peat. It takes time for this new peat to build up, Wils notes. But areas where this happens can work as a nature-based solution to help lift the land.

And people living nearby could visit these natural places when they want to get out of the city.

In the future, the Netherlands may be known for more than tulips, windmills and dikes. “It makes sense to develop a landscape [that] supports the cities,” Wils says. This could stop flooding and provide spots of natural beauty for people to enjoy. “These are actually very easy to combine.”

Wet agriculture isn’t just an option for the Netherlands. It’s also used in Germany, Switzerland and the United States.

No single solution to subsidence will work at every site. But with enough creative problem solving, we can lift up the land to help protect urban areas from undergoing repeated future flooding — not only in Jakarta and Taipei, but also in plenty of other cities across the planet.