Kansai Airport was built on an artificial island — and the island is sinking

Kansai International Airport, opened in 1994, was built on an artificial island in Osaka Bay, Japan. The project cost approximately $20 billion, making it the most expensive airport in history at the time. The island was constructed on soft alluvial clay, and engineers predicted it would settle by approximately 5.7 meters over 50 years.

The settlement was far worse than predicted. By 2002, the island had already sunk over 8 meters, and by the 2020s it had sunk over 12 meters. The differential settlement (uneven sinking) caused structural problems in the terminal building, requiring ongoing repairs and adjustments. The airport's seawalls had to be raised repeatedly to prevent flooding.

The case illustrated the danger of building on soft ground without adequate geological understanding, and how optimistic engineering predictions can lead to catastrophic cost overruns. The airport remains operational, but the ongoing maintenance costs and the risk of flooding from rising sea levels make it a permanent engineering challenge. The case is cited as an example of how geological risk can turn a triumph of engineering into a perpetual liability.