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Kansai Floating Airport: Japan’s Popular Aquatic Airport (That’s Sinking into Osaka Bay)

Long before Dubai started hauling vast amounts of earth into the ocean to create its famed artificial islands, the Japanese were already streets ahead. 

When Kansai International Airport opened in 1994, it immediately became an engineering marvel. Not only was Terminal 1 designed by Renzo Piano with the typical exciting flair that we have come to expect from the man who designed the Centre Pompidou in Paris and the Shard in London, but the very ground below the airport was – well, pretty groundbreaking also.

The artificial island that was created purposely for the airport remains the second-largest man-made island on earth and completely shattered ideas of what was possible. This is an artificial island measuring 4 km (2 1⁄2 mi) long and 2.5 km (1 1⁄2 mi) wide that currently services roughly 25 million passengers each year – and to top it all off, it’s built in one of the world’s most volatile regions for natural disasters, where earthquakes and typhoons are a frequent part of life.

Kansai Airport completely redefined what’s possible and there have been several projects that have followed suit, most notably Hong Kong International Airport which was built on reclaimed land on the island of Chek Lap Kok. But Kansai Airport has a problem and quite a problem at that. The weight of the island and the airport above it is compressing the seabed silts, meaning the island is sinking. By 1999, just five years after its opening, the island had already sunk by 8.2 m (26 ft 11 in) – around 25% more than had been expected. 

With this in mind, the future of this engineering wonder isn’t exactly clear. 

City Rivalry 

When you look at the sprawling mega-metropolis that is Tokyo today, with the Great Tokyo Area now with just over 33 million inhabitants, it’s difficult to see it having any rivals within Japan. It is unquestionably the largest, most populated, densest and most important city in the land of the rising sun, but as a capital city, it’s still a relative upstart. 

tokyo today on shibuya
Shibuya Crossing. By Rodrigo.Argenton is licensed under CC-BY

The seat of government was only moved from Kyoto to Tokyo in 1868 and while it has grown into a staggering urban centre that draws people from around the world, it does still have internal rivals, mainly from the traditional powerful cities to the west in the Kansai region, Osaka, Kobe and Kyoto. 

Kyoto had been the country’s capital for nearly a thousand years, but it, along with neighbouring Osaka, had struggled with investment, trade and tourism in comparison to Tokyo. By the 1960s, the main airport in Osaka which typically supplied the entire region had long reached capacity and local government officials deemed a brand new airport just the thing needed to revitalise the Kansai Region.   


But that was easier said than done. The construction of Tokyo’s Narita International Airport had been met with fierce local resistance which started when those living close to the prospective site found out that they would soon be living next to a massive airport through a segment on the news. You can understand why they would be a little peeved.  

Narita International Airport, Terminal 1 departure entrance.
Narita International Airport, Terminal 1 departure entrance. By Terence Ong, is licensed under CC-BY-SA

Despite the government holding eminent domain – which meant they could seize land that was deemed vital to the public – it was rarely used to its full extent and as the years passed protests increased dramatically. This culminated in a bitter feud where building work continued but protestors began constructing large towers right where the new runways would be. 

The entire process was chaotic and even led to deaths on both sides, the police and protestors. Demonstrations like this were rare in Japan and it proved to be a hellish construction period but the airport did finally open in 1978. 

After watching the bedlam unfold to the east, regional leaders in the Kansai Region were eager to avoid the same kind of controversy. The old airport in Osaka could not be extended and initial plans called for a shiny new airport to be built near Kobe, but this was rejected by the city because they didn’t want it to be open 24 hours a day. 

Instead, planners decided to build the airport on an island in Osaka Bay – with the quite glaring problem that no such island currently existed. An entirely new piece of land would need to be constructed in the water, which sounds like an insanely complex choice, but it would, at the very least, prevent the building site from being inundated with environmental activists like in Tokyo. 

Reclaiming the Land  

The area chosen for the new airport lay 5 km (3.1 miles) off the coast of Honshu, Japan’s largest island, and around 38 km (24 mi) southwest of Osaka. Here the water depth was roughly 18 metres (59 ft) then followed by 20 metres (65 ft) of soft Holocene clay which unfortunately for those looking to build an artificial island directly above is usually around 70% water. 

To compensate for this, 2.2 million sand drains were inserted into where the new island would be. These drainage pipes, measuring 38.1 cm (16 inches) in diameter, were inserted into the clay then filled with sand. Once complete, the pipes were then removed leaving columns of sand to help disperse the water and solidify the material.   

Once this had been done, a seawall was created to mark out the new island’s perimeter. This was created with 69 steel chambers, 22 metres (75 feet) in height, 22 metres (75 feet) in diameter and weighing 200 tons each, which were sunk to the bay floor. The spaces between were then filled with plenty of rock, but also 48,000 tetrapods, those odd-looking concrete installations often included in seawalls and breakwaters, and this initial stage was finished in 1989.

Remember how I said local officials had hoped by building the new airport out at sea they might be able to escape the same unpleasantries as in Tokyo? Well, they didn’t escape scott free because local fishermen in Osaka Bay kicked up quite a storm over the plans and what they saw as a certain infringement on their fishing. But apparently, they were more than satisfied with the financial compensation that was dangled in front of them, because these protests went away fairly quickly.   

Then came the mammoth job of actually filling the seawall perimeter that had been erected. For this operation, three mountains were effectively levelled that brought in a huge 21 million cubic metres (27 million cubic yards) of earth, while the total amount that went in to create the island was an extraordinary 180 million cubic metres (240 million cubic yards) – which is around 100 times the volume of the Melbourne Cricket Ground, which in itself is a gigantic stadium. 

As you’d imagine, this was a staggering operation that lasted three years and included 10,000 workers, 10 million work hours, and eighty ships. But the result was hugely impressive, an island around 10 km 2 (3.8 sq miles) that went down an impressive 30 to 40-metres (100 or 130 ft).  

Sky Gate Bridge-R in Izumisano, Osaka Japan
Sky Gate Bridge-R in Izumisano, Osaka Japan. BY 663highland, is licensed under CC-BY-SA

With the island now complete, all that was needed was a bridge and work began on a 3 km (1.8 miles) connection between the new island and the mainland, specifically at Rinku Town, which itself had been built on reclaimed land some years before. Even this bridge was quite something and upon completion became the longest double-decked truss bridge in the world, with six road lanes above and two rail lines below. It opened in 1991 and reportedly cost a sizable $1 billion to construct, which is around $ 2 billion today. 

Now, before we move on, let’s address the elephant that isn’t so much in the room, but rather slowly sinking in the water. Designers knew full well that the island would sink to some degree, but the question was how much. They went with the gloriously optimistic 5.7 metres (18 ft 8 in) over a roughly 50 year period and we’ll get back to that wildly ambitious prediction a little later in the video.  

The Airport 

Construction on the terminals began in 1991 with Renzo Piano’s beautiful design quickly taking shape. In an attempt to try and cut costs, the local government attempted to convince Piano to reduce the length of the terminal, something he rejected outright. 

renzo piano
Renzo Piano. By Cirone-Musi, is licensed under

In 1988, Renzo Piano won a design contest to create Terminal One at the Kansai Airport. To this day it remains the longest airport terminal in the world at 1.7 km (1.1 miles) in length and includes 42 boarding gates. It has a total gross floor space of 296,043 square metres (3,186,580 sq ft) – roughly the same as Grand Central Station in New York City. 

Its main architectural draw is of course the famed asymmetrical clear-span roof which was designed to look like a pair of glider wings that gradually rise towards the centre of the terminal. The roof was constructed with 82,000 steel panels that are exactly the same shape and size and which rest on the main steel structure beneath.

Rather than having bulky, unattractive air conditioning units placed inside the terminal, Piano designed giant air ducts that blow air up one side of the building and filter through it. The many mobile sculptures hanging from the inside are there not only to look nice but also to make the airflow detectable to passengers.

Several very specific design features were used to try and mitigate the anticipated sinking, but also to strengthen the airport against the inevitable destructive natural elements that have a habit of lurking around this area of the world. The terminals were built using adjustable columns designed to support the building but also to spread out its weight via thick metal plates at their bases. This means that the entire building can essentially be raised, something that is done to a small degree every two years to try and compensate for the sinking of the ground below.     

If Terminal One is the cool, relaxed and stylish Italian slouched over his Vespa, Terminal Two is its spotty teenage brother who still rides his bicycle. That might be a little harsh, but I don’t think anybody would argue with it. While Terminal 1 serves major international routes, Terminal 2 hosts the low-budget carriers, specifically Peach, Spring Airlines and Jeju Air. And no I’ve never heard of them either. 

Terminal 2 has none of the flair or indeed beauty of Terminal 1 and is very much the kind of run of the mill terminal that was built with great haste after World War II around the world. It’s a single storey building that originally had a space of 33,000 sq metres (355,000 sq ft) inside, though that has since doubled with an extension added. It comes with 21 gates and can handle 2.85 million passengers a year on international routes, and 5.5 million domestic customers.


The new airport opened for business on 4th September 1994 and while it certainly proved a success in terms of passenger numbers, it has consistently struggled financially, reportedly losing $560 million in interest every year. Its high landing fees thought to be close to $10,000 for a 747 back in the 1990s ($18,400 today) was the highest of any airport in the world and kept many airlines away. This, along with excessive terminal rent which had been hiked to try and offset the huge cost and hefty utility bills, meant that the new airport quickly found itself in the red. 

Aerial photograph of Kansai International Airport in Japan in 2007.
Aerial photograph of Kansai International Airport in Japan in 2007. By Tdk, is licensed under CC-BY-SA

However, while money was certainly a factor in the early days, it was quickly replaced by a much larger realisation – a sinking feeling if you will. Yes, that’s right, the island was sinking at a much faster pace than first envisioned. By 1990, even before the terminals had been built, the ground beneath the airport had fallen by 8.2 metres (26 ft 11 in) and those brightly optimistic predictions made years before were starting to look not only foolish but dangerously so. 

To combat this, another 3.5 metres (11.5 ft) of earth was added to build up the seawall even further, costing $150 ($313 million today), while the runways were constructed using asphalt rather than concrete to prevent them from cracking in the event of the ground moving slightly. 

But this hasn’t stopped the island from sinking, but rather just slowed it down. The sink rate fell from 50cm a year in 1994 to 7cm a year in 2008, a rate that Japanese authorities claim is manageable. 


Building an extension on an island that is already sinking sounds like a peculiar thing to do, but hey, what do I know. With the airport reaching its capacity during peak hours and a Phase 2 addition already planned out, work began in 1999.  

A second island was added, slightly larger than the first at 1,322 acres, and was completed in 2007 to provide an extra runway for the airport. This was principally done with the IAAF World Athletics Championships in Osaka in 2007 in mind and expanded the airport size to its current 2,600 acres – around three times the size of Central Park in New York. 

There are plans to build a new cargo terminal on the new island but these seem to have been postponed indefinitely by the Japanese government as a way of cutting costs. It’s thought that by 2008, total costs, including land reclamation, terminals, runways and emergency extras to stop the whole thing from disappearing below the waves, added up to a whopping $20 billion (around $25 billion today) – making it one of the most expensive civil engineering projects of all time. 

An Uncertain Future

In April 2001, Kansai Airport was one of ten structures given the “Civil Engineering Monument of the Millennium” award by the American Society of Civil Engineers. A thoroughly well-deserved award that really highlights the technical ingenuity that went into its construction. 

It’s also proving to be a rugged piece of engineering that came through the Great Hanshin Earthquake on 17th January 1995, which killed 6,434 people on the mainland, completely unscathed, despite the epicentre being just 20 km (12 mi) away. Much of this was down to its earthquake engineering and its use of sliding joints throughout the buildings that allow the entire structure to move ever so slightly. 

On 22nd September 1998, the airport was battered by a typhoon with wind speeds over 209 km/h (130 mph) but again came out of it remarkably well. However, things didn’t go quite so smoothly on 4th September 2018 when Typhoon Jebi arrived. The storm caused the seawater to surge and much of the islands were flooded as a result, with water levels reportedly reaching up to the engines of the planes sitting at the airport. That was just about manageable, but the larger tanker slamming into the island’s only bridge in rough seas was not. The airport closed and regular service didn’t fully restart until 1st October 2018, nearly a month later. 

But there is of course another, far larger lingering shadow. While authorities have managed to slow the sinking it’s still happening and most experts agree that there’s little that can be done in the long run. Nobody is quite sure how long the Kansai islands and their airport will remain usable. Some say it’s got another 100 years, some argue that by the 2050s it will have become entirely impractical – while others think it could be as early as the next few years. The problem is also that the islands aren’t sinking at a level rate, with the area around the centre of Terminal One sinking at a faster speed than almost anywhere else. In the last few years, the Japanese government announced plans to raise the sea wall even higher and this is something that might need to become a regular occurrence. After spending $20 billion, this isn’t an airport that can be abandoned anytime soon.  

It’s entirely plausible that a mega-typhoon could already obliterate the islands and if that’s the case, it may already be on borrowed time. This is one beautifully constructed airport sitting above 250 sq km (69.5 square miles) worth of fill that is gradually inching further and further down into Osaka Bay. For this marvellous piece of human ingenuity, the clock is ticking, but how long this countdown may be, nobody quite knows. 

Kansai International Airport – Wikipedia

Renzo Piano’s Kansai International Airport has a mile-long high-tech terminal (dezeen.com)

Kansai International Airport Passenger Terminal Building by Renzo Piano Building Workshop | Airports (architonic.com)

Kansai airport | Institution of Civil Engineers (ice.org.uk)

The Miraculous Floating Airport | Super Structures | Spark – YouTube

The Airport Island: The Story Of Osaka Kansai International Airport – YouTube

Kansai and Kobe International Airport (Osaka Bay, Japan) – 20 Years of SAR – Earth Watching – ESA

The Airport Is Sinking | Flight Today | Air & Space Magazine (airspacemag.com) 

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