FOR most of the year the remote Pacific island of Yakushima is shrouded in black thunder clouds, with annual rainfall of more than 26ft (8m). It is one of the reasons why the island is the setting for an industrialacademic alliance and a revolutionary engineering project: the first water-powered car.

Concrete shafts plunge from the rainforest floor into deep underground caverns and complex machinery rumbles in the belly of a vast mountain covered with 1,000-year-old trees.

The tiny island has been quietly occupied by Honda, the giant Japanese carmaker, to test a vehicle codenamed “FCX”. The Times was allowed an exclusive glimpse of its first day of road trials and a chance to drive the Honda FCX — thought to cost about £1.7 million — which its designers believe could fundamentally change the global car market.

Japanese car companies have far outpaced their European and American rivals in pioneering alternative-fuel vehicles: a secretive race that arises partly from the fact that Japan itself has few natural resources, especially oil.

At the forefront of it all is an intense struggle for green supremacy between Honda and Toyota. Both companies say that they are now on the brink of their ultimate quest: a commercially viable car that does not need petrol or gas products.

The FCX is a giant leap towards that goal because, unlike other experiments with fuel cells, no part of its fuelling or driving process creates anything environmentally unsound.

Despite the hundreds of millions of pounds that have gone into building the FCX prototype, Honda’s top engineer cheerfully tosses the keys for his baby across the all-digital dashboard. Speeding along the perilously twisting mountain roads and the inch of standing water, he bellows orders to slam the pedal down and test its very impressive acceleration. He needn’t have raised his voice; as well as being ultra-green, the FCX is totally silent.

It looks and handles like a normal car, it accelerates explosively and can reach 100mph (161kph), but, for the first time in automotive history, it can do all this without burning fossil fuel and emits nothing more harmful than steam. All it requires to get moving are thousands and thousands of tonnes of rainwater.

Beneath the FCX’s four seats is a hydrogen-powered engine. The gas in its tanks is produced by splitting water molecules, and the power to do that comes from a specially designed hydroelectric plant that takes advantage of the staggering quantity of rain that crashes on to Yakushima.

It is that phenomenon that brought Honda so far from its home base on the mainland to Yakushima. A break in the clouds in the Unesco World Heritage site becomes coffee-shop conversation.

“We have 35 solid days’ worth of rain. Not per year; per month,” the owner of the Azul Hotel grins as he looks out at yet another day of torrential downpour. “It’s our tourist attraction: people come to the island for the rain, and what it does.”

The power of all this water as it cascades off Yakushima’s many mountains and waterfalls was recognised two decades ago, and the island’s electricity needs have long been met entirely by hydroelectric power. But, in a recent development, the Yakushima power company realised that the power plants had enough spare energy to keep lights burning across the island and produce hydrogen with the surplus.

As a gauge of how important the water car development is to Honda, the company has put Yuji Kawaguchi, its senior chief engineer, in charge of the FCX project. With the odd complaint that the weather was better at Honda’s research centre in California, Mr Kawaguchi rides a mining-car into the depths of one mountain to show off his discovery.

Amid deafening turbines and the thunder of millions of gallons of water forced into narrow tubes, the engineering veteran of Honda’s Formula One motor racing campaigns explains sheepishly that, at the moment, it takes about 12 hours to produce a useful quantity of compressed hydrogen — but his car can go for about 250 miles on a full tank.

Both Honda and Yakushima Electric have got the scent of a futuristic, hydrogen-powered society and have grand plans. Deeper into the mountain, work has started on a hydroelectric turbine that will be devoted to producing hydrogen. Mr Kawaguchi scribbles some enormously complicated sums on a scrap of paper and concludes that eventually it would be viable to have all the 9,500 vehicles on Yakushima running on water-generated hydrogen.

Some miles away, the hydrogen filling station is also part of the experiment. Kagoshima University and the United Nations University are participating in what is a critical aspect of the whole project: for hydrogen cars to be viable, it has to be easy to fill them up. Even Honda’s senior chief engineer is not yet qualified enough to perform the three-minute task, which involves first earthing the car with a thick metal wire before attaching the gas pump with an airtight seal.

Mr Kawaguchi accepts that there are many issues that need to be resolved before the world goes water-powered, or even hydrogen-powered. The durability of certain key parts of the engine is still not up to commercial scratch; there are safety questions over hydrogen stations; and the cost of each vehicle is still prohibitive.

But asked if he believes that the FCX is the start of something big, Mr Kawaguchi is unflinching: “This is the future, no doubt about it. How we actually produce the hydrogen is an interesting question, and this hydroelectric plant is just one experiment — but hydrogen-powered cars are within our grasp.”

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