Many cities around the world are sinking because too much water is extracted from the groundwater reservoirs beneath them.

This sinking makes these coastal cities more exposed to rising seas as the climate warms.

But as our new research shows, this process can work in reverse; when groundwater is replenished, cities can rise.

Our findings also reveal an unexpected clue: faultlines play a key role in shaping where the fastest groundwater recovery and land uplift occur.

Land subsidence, the gradual sinking of the ground surface, affects many of the world’s major cities.

Parts of Jakarta, the capital of Indonesia, are sinking more than ten centimetres each year, prompting the government to plan a relocation of the city.

The coastal city of Tianjin, China, is home to 15 million people. It, too, is a subsidence hotspot. If sinking there continues unabated, 15% of the city’s population will be underwater by 2120.

Similar stories are playing out from San Diego to Iran’s major cities, wherever groundwater has been pumped faster than it can refill.

In Aotearoa New Zealand, recent studies show around 80% of the urban coastline is sinking, and that groundwater may be a contributing factor in some areas, including Christchurch and Wellington.

Many cities are built above natural groundwater reservoirs (aquifers), and are literally held up, in part, by the water beneath them.

Think of a gigantic water balloon beneath a city. The balloon represents an aquifer full of water. When groundwater is pumped out, the “balloon” deflates, and the ground above sinks.

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