In the early 1990s, a group of engineers in Japan embarked on an ambitious project to solve a growing problem -- how to expand air traffic capacity in a densely populated urban region with limited space.
The solution was groundbreaking: a massive, man-made island constructed in open water, designed to withstand the test of time and nature, rivaling other unique artificial islands that you can visit.
With cutting-edge technology and meticulous planning, expectations were high. But as years passed, unforeseen challenges emerged, pushing the limits of engineering predictions.
What was once seen as an architectural triumph is now facing a relentless battle against the forces of nature. Kansai International Airport, the world's first offshore airport, is sinking faster than anyone had anticipated. This persistent issue threatens the airport's operations and longevity, making it a focal point for engineering discussions worldwide.
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Sinking Faster Than Expected: The Science Behind Kansai Airport's Troubles
Japan's sinking airport was built on reclaimed land
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Kansai International Airport (KIX), opened in 1994, in Osaka Bay. The project was heralded as an engineering triumph, blending innovation and ambition. It's one of the many stunning airports that travelers hope to have a layover in. But beneath the airport's bustling terminals lies a problem that has escalated faster than engineers expected -- its foundation is sinking at an alarming rate.
Originally projected to subside eight meters over 50 years when it opened in 1994, the island sank over 12 meters in its first eight years. As of 2023, the total subsidence had reached 13.61 meters, and it continues to descend by six centimeters annually.
"When the Kansai airport was constructed, the amount of soil to reclaim the land was determined based on necessary ground level and subsidence estimation over 50 years after the construction." - A representative of Kansai Airport, Yukako Handa, said in 2018 in a Smithsonian Magazine interview
The main reason for the Kansai Airport sinking is the type of natural material under the airport's man-made island. The clay layers beneath the reclaimed land are "compressible," meaning they can be easily compressed or squished under pressure.
When heavy structures like the airport are built on top, the weight causes the clay to compress, which leads to the sinking of the land over time. Essentially, the soft, squishy clay isn't able to support the weight of the airport as well as solid ground would.
Kansai International Airport
While engineers anticipated gradual settling and designed accordingly, the reality has been far harsher. Millions of liters of water were drained from the seabed during construction, compacting the soft clay into a denser state. However, deeper clay layers settle unpredictably, contributing to faster subsidence than initially forecast.
Efforts to counteract this issue have been extensive and expensive. More than $100 million has been spent on maintaining the airport's elevation, including raising runways and fortifying its seawalls.
Despite these measures, the constant sinking requires ongoing intervention, posing logistical and financial challenges for operators.
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The Challenges Of Nature & Infrastructure Resilience At Kansai Airport
Natural disasters continue to test Kansai's durability
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Beyond the issue of subsidence, Kansai Airport contends with the threat of natural disasters. Japan is one of many countries prone to earthquakes. Just after welcoming the 2025 new year, a powerful 6.9 magnitude earthquake struck southwest Japan, prompting an immediate tsunami warning too.
In 2018, Typhoon Jebi caused extensive flooding, temporarily shutting down the airport. This highlighted vulnerabilities in the airport's defenses, prompting upgrades such as elevated runways and reinforced seawalls.
The airport also survived the 1995 Great Hanshin Earthquake and subsequent typhoons, a natural disaster that contributed to Kansai Airport and nearby Kobe becoming one of the fastest-sinking cities worldwide.
Yet, each disaster adds pressure to an already sinking foundation. Engineers continue to innovate with improvements to the airport's infrastructure, but questions remain about its sustainability in the face of future challenges.
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Kansai's Role In Japan's Aviation Future
Kansai remains pivotal despite its challenges, with expansion plans in progress
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Despite its difficulties, Kansai Airport remains integral to Japan's aviation network. Handling nearly 26 million passengers in 2023, the airport is a hub for international travel and commerce. It is also a gateway to the Osaka Expo 2025, underlining its importance on the global stage.
Recent renovations, including a new international departure area, reflect ongoing investment in its future. Experts, however, caution that continued sinking may necessitate even greater interventions in the coming decades.
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Kansai International Airport is a testament to human ingenuity and determination, but its ongoing battle with subsidence highlights the challenges of building on reclaimed land.
As engineers work to stabilize the airport and mitigate risks, its story serves as a reminder of the unpredictable forces beneath our feet.