Year after year, centimetre by centimetre, Rudi Suwandi's home in north-west Jakarta is being swallowed.

The trees and park where kids would play have vanished, houses with their own backyards - gone, entire rooms of Rudi's house have been consumed. Even the village cemetery where his ancestors and thousands of others are buried has disappeared.

In its place lies stagnant, murky water.

Rudi first remembers the water appearing in the mid-90s, when the village started flooding every rainy season. Every year, it would get higher and take longer to drain away.

In 1995 it was 20 centimetres deep and took three to four months to dry out. By the early 2000s his village, Kapuk Teko, was permanently flooded.

Now, entire areas are submerged two metres underwater. All the houses are perched on stilts above the lake, connected by gangplanks and concrete paths. The only way into Rudi's village is via a low, narrow concrete bridge.

"The places where we were born, played and grew up, all are gone," Rudi said. "Everyone here feels deeply sad about what's happened to our village, the neighbourhood with trees and land to play has all gone."

For years, the ground beneath their village has been sinking. And it's not just here. It's happening right across Jakarta.

Roads are warping, breakwalls have been swallowed and entire buildings have been left abandoned.

Worst of all, the city, which straddles the Java Sea and serves as the exit point for a handful of major rivers, is more prone than ever to flooding.

Researchers at Indonesia's Bandung Institute of Technology (ITB) have been at the forefront of tracking the phenomenon, known as land subsidence, for more than two decades. Using a mix of satellite radar technology and in-ground measurements they've been able to map the rate at which the city is sinking.

Land subsidence in Jakarta since 1977 Subsidence (metres) 0 -5 INDONESIA SHALLOW DEEPER Jakarta 1977 1997 2007 PROJECTION PROJECTION 2017 2020 2025 PROJECTION PROJECTION 2035 2050 Source: Bandung Institute of Technology Land subsidence in Jakarta since 1977 Subsidence (metres) 0 -5 SHALLOW DEEPER 1977 1997 2017 2007 PROJECTION PROJECTION 2020 2025 PROJECTION PROJECTION 2035 2050 Source: Bandung Institute of Technology

Since the 1970s, parts of Jakarta have sunk more than four metres, at a rate of up to 25 centimetres a year. That means these areas are sinking faster than any other city in the world.

The subsidence of the city combined with climate change-induced sea level rise creates a perfect storm of natural disaster.

"If we model this and project forward, around 95 per cent of northern Jakarta's surface in 2050 will be under the sea," said Heri Andreas, an expert on land subsidence at ITB.

"That means if we don't protect [Jakarta] with a seawall or other means, then Jakarta would be inundated by sea water."

Without major intervention to prevent sinking, Dr Andreas' research team predicts that more than a quarter of the city could be swamped by the sea by 2025. The north of the city, an area home to more than 2 million people, is especially vulnerable.

To understand why this is happening at such a rapid rate in Jakarta it helps to look at just how densely populated the city is.

The urban, built-up area of Greater Jakarta is estimated to be home to more than 34 million people in an area two-thirds the size of Sydney's urban area.

In that area of Jakarta, more than 10,000 people live in each square kilometre. By comparison, built-up Sydney has about 2,000 people per square kilometre.

How does population density in Jakarta compare to other cities? The urban reaches of Jakarta are among the most densely populated in the world, with more than five times the number of people per square kilometre than Melbourne or Sydney. 1 person JAKARTA 10,200 SYDNEY 2,000 LONDON 5,600 MELBOURNE 1,600 1 square = 1km2 Source: Demographia How does population density in Jakarta compare to other cities? The urban reaches of Jakarta are among the most densely populated in the world, with more than five times the number of people per square kilometre than Melbourne or Sydney. 1 person MELBOURNE 1,600 SYDNEY 2,000 LONDON 5,600 JAKARTA 10,200 1 square = 1km2 Source: Demographia

The data paints a staggering picture of consistently high density across large parts of Jakarta. As a result, the city is buckling under the pressure.

Jakarta's traffic gridlock is infamous; it regularly ranks among the most congested cities in the world. This congestion contributes to the city's poor air quality.

On top of that, only a tiny portion of the city's wastewater is treated. Much of it pours into the city's vast networks of rivers and canals.

Satellite imagery captured each decade from the 1970s shows just how much the city's urban reach has expanded.

Jakarta from space Jakarta from space The green colour surrounding Jakarta highlights healthy vegetation, much of which has disappeared since the 70s. 1978 1989 1999 2009 2018 Source: NASA/USGS Landsat

For the most part, infrastructure has not matched the speed of growth.

Large parts of the city do not have access to or cannot afford piped water. Instead, construction sites, shopping centres, high-rise apartments and residents are extracting huge amounts of water, often illegally, from the aquifers below Jakarta.

Each year the city pumps around 630 million cubic metres of water from the ground. This is the main reason the city is sinking.

"The formula is simple... if we can stop using the groundwater, the subsidence will stop," Professor Andreas said.

"The question is, can we stop using groundwater?"

Other major cities have done it before. Tokyo, a city of 38 million people, sank more than two metres in the 20th century before measures were put in place to restrict the mass extraction of groundwater. The city's water supply was also largely shifted to surface water.

The changes stopped the Japanese capital from sinking into the ground.

But experts believe not enough is being done in Jakarta to stop the subsidence.

Indonesia's President Joko Widodo has overseen heavy investment in infrastructure and flagged earlier this year that improving Jakarta's water infrastructure would be part of a US$40 billion investment in the city over the next decade.

The city's governor also toughened policies to crack down on the illegal use of groundwater. But it's unclear what impact this will have on subsidence because of the lack of reliable methods to monitor deep groundwater levels.

Construction of the "Great Garuda Sea Wall" began after devastating floods in 2013 sharpened the focus of policy makers on the damage subsidence could cause. The plan was to build an elaborate US$40 billion district on the water in the shape of a garuda, a mythological bird-like creature, to act as a buffer from the Java Sea.

But after concerns the structure would create a giant toxic bay filled by Jakarta's heavily polluted rivers, and a corruption scandal linked to some of the islands in the project, the concept was scrapped. Work continues on strengthening walls in northern Jakarta, as well as a far less ambitious sea wall project.

And, of course, there was the announcement that Indonesia would be looking for a new home for the Indonesian capital away from the island of Java to ease some of the region's population problems. It's a plan that has been raised before and never came to fruition. But if it does go ahead this time, it's tipped to take at least 10 years.

Despite the setbacks and drawn out timelines, experts are hopeful those in charge will see the need to change because the city is running out of time.

"We are able to do this. We have the technology. We just need the will of stakeholders because that first step is the hardest one," Heri Andreas said.

For now, all that lies between the communities to the north of Jakarta and catastrophe, are thick concrete walls like this.

But even they are sinking.

On one side of the sea wall, a bay where plants grow from the skeletal hulls of half sunken boats and children frolic in the water.

On the other side, several metres below the water level, tightly packed neighbourhoods connected by a maze of narrow alleyways only wide enough for a motorbike.

Here in Gedung Pompa, named after the flood pump next to the community, there's been no major flooding since 2013, when the wall was rebuilt a metre higher than before.

With her baby attached to her hip, Devi Mariani recalls the terror of what happened last time the water breached the wall.

"I was heavily pregnant and about to go into labour when the flood happened," she said.

"It was as high as our neck. They evacuated me to the hospital but cars and ambulances couldn't reach me so they had to take me out in a makeshift raft."

As her six-year-old girl plays with her friends, Devi's mother Jumiati watches on with a smile.

"Now the kids run around freely. They can play wherever they want to. Even if it rains hard, the water drains away easily."

If she knows the wall is just a temporary fix, as the ground sinks imperceptibly around her, she's giving nothing away.

Credits

Reporting: Mark Doman, Ari Wu and David Lipson

Additional research and interviews: Chicco Guilianno and Anne Barker

Photography: Ari Wu and Phil Hemingway

Design: Alex Palmer

Notes about this story

The population and density data used in this story is based on the Demographia World Urban Areas 2019 report. It uses mapping software to estimate continuous areas of urbanisation rather than using city boundaries. This is combined with local census data and UN population projections to reach current population and density estimates.