An early glimpse of the 21st century automobile came not from Henry Ford or a Formula One pit lane, but from the pen of Jules Verne. "The energy of tomorrow is water broken down into hydrogen and oxygen using electricity," he wrote in The Mysterious Island in 1874. "These elements will secure the earth's power supply for an indefinite period."
After more than a century of the internal combustion engine, car makers are now ploughing billions into developing alternative fuel technologies to reduce dependence on finite oil. The most promising contenders borrow from Verne's vision, using hydrogen fuel cells - a technology discovered in 1839 - to turn the wheels and leave nothing but water vapour in their wake.
The need to rethink the car is long overdue. The 210 million vehicles in the US spew almost 1.5 billion tonnes of greenhouse gases into the atmosphere every year and, according to a US environmental protection agency report, the latest models average little more than 20 miles to the gallon - the worst showing since 1980. Even George Bush, diehard Texan oilman, acknowledges that identifying workable alternative fuels has become imperative.
Now, with everybody from American "sports utility vehicle" nuts to Mexico City cab drivers likely to be bowling along in eco-friendly vehicles by 2010, a bewildering array of technologies is emerging, each claiming to revolutionise the automobile.
Here we take a look at seven of the best, from vehicles already moving into production to space-age prototypes that may never make it into the showroom, but offer a blueprint of how cars may look and move over the next two decades.
Honda FCX
Certified for everyday use in the US
Appearance: Ordinary city runabout, two doors, four seats
The technology: Despite its ordinary looks, the FCX is the first hydrogen fuel cell car to be "government-certified" for everyday use in the US: it's the first real-world green car - almost.
You can't buy one yet, but you can rent one in Tokyo and, by the end of the year, in Los Angeles.
As with other hydrogen fuel cars, it uses tanks of compressed hydrogen. When pumped into the fuel cell, the hydrogen heats up and each molecule is split into two positive ions and two electrons. The electrons are directed into a circuit to drive the engine, while the ions pass through a membrane where they combine with oxygen ions produced from outside air. When the ions combine, they form water vapour, the only emission.
Fuel cell technology has been around since the 19th century but companies have struggled to make it efficient. It remains to be seen whether the technology is commercially competitive.
The first problem is that hydrogen is obtained by using electricity to split water into hydrogen and oxygen, or by stripping the carbon from hydrocarbons such as natural gas, and this takes energy. Ideally this energy would come from renewable resources, but in most regions, that's not an option right now. So it's less green, and more costly - compressed hydrogen costs the equivalent of $3 a gallon. And we lack pumps to fill up at.
The figures: Over €6,000 a month to rent in Japan, its top speed is 93mph and it will go 220 miles before needing a top up. It's officially a "zero emission" car.
Who'll drive them? If they can get the price right and build enough filling stations, it'll be perfect for environmentally aware urbanites. Right now it's a good option for environmental consultancies and other green businesses.
GM Hy-Wire
No steering wheel, no pedals - a giant skateboard
Appearance: From the outside, conventional enough: silver body, four wheels, windows, etc. Hy-Wire's massive windscreen gives superb visibility.
The technology: It does away with the two basic components of traditional technology: no engine under the bonnet and no hydraulic or mechanical linkages beneath the chassis. Everything that propels and steers the car is housed inside the 28 cm-thick aluminium chassis at the base of the vehicle - "the skateboard". Power comes from 200 hydrogen fuel-cells, giving a top speed of about 100 mph. The hydrogen gas fuel is in three cylindrical tanks of a carbon composite.
Instead of steering wheel and pedals, a "by-wire" steering grip is linked to a central computer in the skateboard. To accelerate, just twist the grip; to brake, squeeze it. To steer, push the grip up or down. The electronics are by Swedish aeronautics company SKF.
The figures: GM has spent about about €14 million on the Hy-Wire, using 500 specialists. It has more than 30 patents on the technology. Biggest weakness is a range of only 75 miles before it needs more gas.
Who'll drive them? GM says it will be producing about 100,000 by 2010, and everyone from "white van" men to boy racers will want one. Because all the technology is in the skateboard, the car's shape can be changed to suit the owner. For instance, the passenger compartment can be replaced with a van or sports car shell.
Hypercar Revolution
Super-sleek, super-light, heavy price tag
Appearance: Cross between a luxury sports car and a super-sleek family hatchback. Best in silver.
The technology: Hypercar has just won the environment category in the World Technology Awards. Most future-car makers focused on developing fuel cells powerful enough to drive existing cars, but Colorado-based Hypercar has done it the other way - redesigning from the tyres up to produce a car that's light and efficient enough to be driven by fuel cells.
The Revolution is eight times more efficient than normal cars. The body is made of a composite of carbon fibres set in a plastic matrix. With just 14 main components, the body is much simpler to assemble than today's stamped and welded metal bodies which can run to 250 pieces.
The car weighs only 907 kilograms, but the body materials, kilo for kilo, are five times stronger than steel, improving safety. Weight is further cut by special low-rolling resistance tyres developed with Michelin - they cut down friction, which accounts for a third of a car's fuel energy. The absence of a standard engine, starter, alternator, clutch and transmission reduce bulk even more. So, the Hypercar needs a fuel cell that's a third of the weight of those being developed by traditional makers.
The figures: The Revolution is capable of the equivalent of 99 miles per gallon of petrol with a range of 330 miles on 7.5 lb of hydrogen. It's 0-62 miles per hour time is 8.3 seconds.
Who'll drive them? Put it this way - when in rolls out in 2005, it will have a price tag similar to luxury four-wheel drives such as the BMW X5 and the Mercedes M320.
Mercedes F-Cell
Complete with go-greener stickers on the side
Appearance: A standard Mercedes A-Class with unsubtle stickers up the side to make sure other road users realise how technologically advanced you are.
The technology: The 60 cars unveiled last October represent the first fleet of vehicles powered by pure hydrogen to be made in near-standard production conditions. The cars run on compressed hydrogen funnelled into 440 fuel cells under the floor. DaimlerChrysler claims it's completely emission-free and extremely quiet.
The figures: Daimler-Chrysler is spending billions developing alternative fuel technologies and this is the leading candidate. The F-Cell has a range of about 95 miles (before it needs a new tank of hydrogen) and a top speed of about 100 mph. Its 0-62 mph time is 16 seconds. Like most hydrogen cars, consumption puts petrol and diesel cars to shame - it's equal to more than 100 kpg.
Who'll drive them? Consumers in the US, Europe, Singapore and Japan are already road testing, while UPS is testing one as a distribution vehicle.
Chrysler Natrium
Fuelled by borax, a naturally occurring mineral
Appearance: Same as Chrysler's swish Grand Voyager people carrier, used to move the Blair brood around Britain.
The technology: Power is by hydrogen fuel cell, but the hydrogen source makes the Natrium special: sodium borohydride, basically borax laundry detergent combined with hydrogen.
Chrysler says that sodium borohydride is widely available in the US, is non-toxic, non-flammable and can be recycled - an endless fuel supply?
Hydrogen gas is extracted from hydrated sodium borohydride pellets. The only residue is recyclable borax. Refuelling would entail simply keeping about a kilo of pellets and two gallons of water in the vehicle.
Performance is impressive; its 480km-range is roughly equal to a petrol engine. Natrium is heavier than its conventional cousin, but Chrysler says it can shave weight if it goes into production.
The figures: Top speed is 80 mph. Consumption is equal to 30 mpg of petrol.
Who'll drive them? Eco-conscious, squeaky-clean heads of government.
Nuna
Solar car powered by cast-offs from the Hubble space telescope
Appearance: A waist-high Jetsonian powerboat on three wheels.
The technology: Built by eight Dutch students, it's reigning champion of the World Solar Challenge, a 3,010-km race across the Australian desert. Among the most advanced vehicles on earth, the Nuna's outer shell is made of space-age plastics while the main body is carbon fibre, reinforced with Kevlar.
Power comes from 36 solar cells that coat the shell, some of which have been in space as part of the Hubble telescope, provided by the European Space Agency. The car also carries maximum power point trackers, like those developed for ESA's comet-chasing Rosetta mission, which guarantee optimal balance between battery power and cell power.
The figures: Nuna has a theoretical top speed of more than 160kph - although in the race, which it completed in a record four days, its top speed was just over 100 kmh.
Who'll drive them? Nobody but Dutch students and astronauts. Nuna will never become a production car, but it will influence future vehicles.
e.Volution
Runs on air alone
Appearance: A squished Nissan Micra; something Noddy would drive in the urban transport crisis.
The technology: From Zero Pollution Motors in France, it's likely to be the first air-powered car in mass production. The e.Volution has a unique two-cylinder compressed air engine, designed by former F1 engineer Guy Negre. Compressed air stored in tanks under the chassis is fed through an injector to a small engine chamber where it expands, pushing down on the pistons to move the crankshaft.
According to ZPM, the 700-kilo car will be able to run for up to 10 hours at an average speed of 55mph before recharging. But it needs electricity - currently from fossil fuel - to compress the air.
The figures: The e.Volution will be able to travel 193 km on about 30 cents worth of compressed air. Technical issues have delayed the production date, but it will retail for about €8,800.
Who'll drive them? Mexico City taxi drivers, for starters. The Mexican government has signed a deal to buy a fleet to replace petrol and diesel taxis.