Shanghai is one of the largest cities in the world and is famous for many reasons. We’re sure you already know about this incredible Chinese city. But there is something many people still don’t know about it: the ground beneath the city has been slowly sinking for decades… Yes, an incredible process known as land subsidence happens when too much water, oil, or gas is taken from underground. As these materials are removed, the soil becomes compressed. Over time, this can damage streets, buildings, and flood defenses. But it also makes the city more vulnerable to rising sea levels, and as you may imagine, that’s a big problem…
In this context, engineers have found a very surprising way to slow down — and sometimes slightly reverse — this sinking. Understanding how they do it, it helps explain how Shanghai has managed to stay stable despite these challenges!
Shanghai is sinking… But why?
Let’s explain this because it sounds very frightening. What happened is that many people first notice this problem through small everyday signs, doors may stop closing properly, cracks appear on walls, and streets flooded more easily during rainy seasons… Back in the 50s and 60s, Shanghai was sinking by as much as 17 centimeters every year because too much groundwater was being pumped from the clay and sand layers under the city.
Geologists and experts often compare the soil underground to a firm sponge, those tiny spaces between soil particles hold water, oil, and other fluids. When these fluids are removed faster than they can be naturally replaced, the sponge gets squeezed. As a result, the ground above slowly sinks… Over time, as you may imagine, this affects buildings, roads, river walls, and other important parts of a low-lying city like Shanghai.
Engineers to help
The solution used in Shanghai is simple and clever, instead of taking water out of the ground, authorities reduced pumping and began putting treated river water back underground through special wells. Why? Because this helps increase pressure beneath the surface and keeps the soil more stable! Incredible, huh?
Something similar was used in Long Beach (California). There, heavy oil extraction in the mid-20th century caused some areas to sink almost nine meters!! But engineers injected treated water into the oil fields and even lifted some parts slightly.
Measures taken in Shanghai
Let’s have a look at the measures taken in Shanghai:
- Installing recharge wells to inject treated river water
- Reducing groundwater pumping
- Using deeper aquifers for water withdrawals
Incredibly the average sinking rate dropped from 17 centimeters per year to around 1 centimeter per year, wow. Streets, subway tunnels, and riverside flood defenses still move a little, but the changes are now slower and easier to manage.
Are there limits and risks?
Obviously, even injecting water helps, it is not a perfect solution. Much of the underground soil has already compacted permanently, so the city cannot fully recover the height it has lost. Experts describe this method as a braking system — it slows the sinking but cannot completely reverse decades of damage.
If the process is not done carefully, small earthquakes could happen, or maybe water might move into sensitive underground areas… Treating and pumping large amounts of water requires significant energy and cost
What are engineers doing?
To manage and reduce these risks, engineers are constantly monitoring the land using GPS, satellites, underground sensors and other systems. These tools allow them to track ground movement and water pressure in real time and make sure everything remains safe.
One of the biggest cities…
In a city like Shanghai, even a few extra centimeters of elevation can make a big difference… It can mean the difference between floodwater staying near the river or rushing into subway tunnels during a storm. By carefully managing underground fluids, the city gains valuable time to:
- Improve levees and flood defenses
- Redesign drainage systems
- Protect important infrastructure
- Plan safer areas for residents
Shanghai’s experience shows that even a serious problem like a sinking city can be managed through thoughtful engineering and careful planning. It is an example of how innovation and responsible resource use can help cities face major challenges and prepare for the future.