Satellite maps show sinking ground beneath major US cities

The ground beneath many major US cities is now known to be subsiding across broad swathes, changing assumptions about urban stability in places once thought secure.

That realisation recasts everyday hazards: gradual, out-of-sight movement can quietly load extra stress on infrastructure and intensify pressures that already shape city life.

A map of sinking ground

A national picture is taking shape in which sinking ground spans whole built-up districts, rather than being confined to a handful of isolated trouble spots.

New satellite mapping traced this movement street block by street block, showing downward shifts that can build year after year before any problems are obvious at ground level.

At Virginia Tech (VT), Dr Leonard Ohenhen recorded these changes by measuring how the land under heavily used cities shifted steadily over time.

The record indicates the movement continued across multiple years, reaching both known hotspots and neighbourhoods that had previously been treated as stable.

By showing how extensive and uneven the subsidence has become, the results lay the groundwork for a closer look at the drivers behind it and the risks that follow.

The numbers behind the drop

Looking across 28 large cities, the maps detected land subsidence - a slow sinking of the ground surface - in every single one.

During 2015-2021, at least 20 percent of each urban footprint was moving downwards, and in some places the share was substantially higher.

Using 2020 census blocks to estimate population, the researchers calculated that roughly 34 million people lived on land that was sinking.

Because inland cities showed the same trend, the challenge is not limited to coastal flooding; it also has implications for routine urban planning.

Satellites that measure motion

Radar satellites revisited the same city blocks again and again, enabling researchers to track ground changes at the scale of millimetres.

By comparing radar images through time, the team converted slight differences in the returning signal into readable maps of movement.

To maintain consistent coverage in cloud, at night, and in other difficult conditions, the analysis used Sentinel-1 radar from Copernicus.

Those repeat observations produced fine-grained data, allowing planners to assess whole neighbourhoods instead of relying only on scattered survey points.

Pumping water, sinking land

In many cities, rising water demand has driven deeper groundwater pumping, and the land above those wells has responded by settling.

As water is removed from an aquifer - an underground layer that holds water in its pore spaces - the grains can pack more tightly and the total volume reduces.

That compaction can be partly irreversible, meaning even wetter years may not restore the land to its former level.

Cutting back withdrawals can slow the process, but doing so depends on sustained monitoring of water levels and long-term enforcement.

Ice age effects still unfolding

Subsidence was not always tied to pumping, because some cities sit on ground that is still adjusting after the loss of ancient ice.

This ongoing process, called glacial isostatic adjustment, describes the slow response of the Earth’s surface after ice melts, lowering certain regions a little each year.

The maps showed consistent downward movement in cities including Chicago and New York, even where local pumping was relatively limited.

Because these broader forces cannot simply be switched off, planners often have to focus on adapting buildings and drainage rather than trying to stop subsidence.

Cities compress the surface

Urban expansion can add its own load: the weight of development can gradually compress soft soils and made ground.

Building work can also alter how groundwater moves, raising pressure in one area while nearby layers dry out and compact.

These differences matter because one neighbourhood might rise slightly while another drops, creating uneven gradients across roads and buried services.

The outcome can be a patchwork of movement that places stress on pipes and rail lines, even when the citywide average appears modest.

Uneven motion, real damage

The greatest threat to infrastructure came from uneven movement, as many structures can cope with steady sinking far better than with bending or tilting.

Engineers refer to this as differential settlement - uneven ground drop over short distances - and it can distort foundations and lead to cracked walls.

Working with neighbourhood-scale grids, the study identified more than 29,000 buildings located in zones with elevated risk of damage.

Being flagged as higher risk did not mean failure was inevitable, because soil properties, foundation design and maintenance still affect what happens.

Flooding follows low spots

When land sinks, water pathways can shift, and even a small drop can direct runoff into newly formed low areas.

“\“A lot of small changes will build up over time, magnifying weak spots with urban systems and heighten flood risks,\” said Ohenhen.”

Flood planning that overlooks land motion may miss where the next vulnerability will appear, particularly in flat districts with poor drainage.

Planning that reduces risk

Since much of the subsidence is connected to water use, cities can slow it by managing withdrawals and replenishing basins.

Building regulations can also anticipate uneven settlement, specifying foundations and joints that tolerate gradual movement without breaking down.

Regular mapping can help direct inspections towards bridges, roads and pipelines that cross the most rapidly changing ground.

Even with stronger planning, parts of many cities will continue to move, making ongoing risk management essential.

A slow hazard, revealed

Taken together, the results shift subsidence from an abstract concern to a measured, citywide pattern that can influence everyday urban life.

Long-term safety will rely on combining water management, building design and routine monitoring - while recognising that some settling is likely to persist.

Credit image: ESA.