For decades, scientists have warned that Shanghai, one of the world’s largest and most densely populated coastal megacities, has been slowly sinking because of how humans have used the land beneath it. The problem is not new. As per reports, historical records show that parts of Shanghai have dropped more than 2 metres over the past century because of excessive groundwater pumping and the city’s soft, compressible soil. That sinking makes the city more likely to flood, have sea levels rise, and have its infrastructure damaged.But even with all of this sinking, Shanghai hasn’t suddenly fallen apart or disappeared. Engineers and officials have found ways to slow down how quickly the ground sinks. They have started doing things like pumping less groundwater, moving water extraction to deeper aquifers, and recharging aquifers with treated surface water instead of letting the land get drier as they remove water. These actions have been like an invisible support under the city, pushing back against subsidence and slowing it down.Some parts of Shanghai are still settling, but the average rate of subsidence has dropped a lot since it was at its highest in the middle of the 20th century. Understanding how and why this happens involves looking deep into the ground, into porous sediments and human-managed water systems that together define the risks and responses for this megacity.
What causes a city like Shanghai to sink
Shanghai sits on the flat, soft sediment of the Yangtze River Delta. These sediments were laid down over thousands of years and contain layers of silt, clay and sand that behave much like a sponge when loaded or unloaded with water. When those tiny pore spaces are filled with water, they help support the weight of buildings, roads and soil above. But when too much fluid is withdrawn, the supporting pore pressure falls, and the sediment compacts under its own weight. This process leads to land subsidence, or sinking of the ground surface.In the early and mid-20th century, rapid industrial growth and population expansion led to heavy pumping of groundwater in Shanghai. People extracted water for industry, agriculture and growing urban needs. Combined with the weight of tall buildings, this contributed to land sinking atrates that in some areas reached or exceeded 10–15 centimetres per year in the late 1950s and early 1960s.Subsidence is not unique to Shanghai. Other cities, including Mexico City and Long Beach in California, have experienced similar ground settlement because of fluid withdrawal from beneath the surface. In each case, the behaviour of fluids in subsurface sediments plays a central role in whether the ground sinks, rises or remains stable.
The invisible support: How water injection helps
If removing water causes the sediment to compact, scientists and engineers asked a simple question: What happens if we put some fluid back? Rather than treating groundwater levels as something to drain, cities began experimenting with injecting water back into depleted aquifers and old oil or gas zones. Injecting water under pressure increases the pore fluid pressure in the sediment, providing additional support and reducing the rate of compaction.In Long Beach, California, a water injection programme begun in the late 1950s helped reduce land subsidence from as much as nine metres across the region to much lower rates. Engineers used treated seawater and produced formation water injected through hundreds of wells to slow the compaction of depleted layers. As per reports, that programme is widely cited as one of the first large-scale applications of fluid injection to manage subsidence.Shanghai adopted a related but slightly different approach. Authorities gradually cut back excessive groundwater pumping, shifted water withdrawals to deeper layers, and installed recharge wells that inject treated river water into the subsurface. As a result, the once rapid subsidence has slowed to roughly one centimetre per year in recent decades. This does not mean the city is rising again, but it does mean that the speed of decline has been reduced significantly.
Why reducing subsidence matters
For a coastal city like Shanghai, even a few centimeters of ground movement can make a big difference. Land subsidence, along with rising sea levels due to climate change, makes flooding more likely, damages infrastructure like subways and roads, and raises the cost of flood defenses. In real life, every centimeter of less subsidence gives planners and engineers more time to make drainage better, levees stronger, or rethink how infrastructure is built.But experts are careful to say that fluid injection and artificial recharge are not cures. A lot of the compaction that happened before these practices started is pretty much permanent. Scientific studies of subsidence in places like Mexico City show that the ground rarely goes back to its original height, even when groundwater is allowed to rise.There are also risks associated with fluid injection, including the possibility of reactivating faults or triggering small seismic events if water is added too quickly or in inappropriate layers. For this reason, modern programmes rely on detailed monitoring systems, including GPS, satellite radar and borehole instruments, to track very small changes in ground level and underground pressure.
The path ahead for Shanghai and other sinking cities
Across China and beyond, many low-lying megacities face similar challenges. Historical records show that Shanghai’s central districts have sunk by more than two metres since the early 20th century because of a combination of groundwater extraction and the weight of urban development.Government agencies and researchers now monitor ground movement carefully and are integrating lessons from other cities to manage subsidence. Techniques such as artificial recharge and careful groundwater management are becoming part of broader urban planning strategies designed to reduce long-term risk.While Shanghai has not sunk entirely, its experience and that of other subsiding cities illustrate how deep beneath our feet, invisible processes can shape the fate of entire metropolises. Understanding and managing those processes remains a key priority as sea levels continue to rise and cities grow larger.
