Cars that run on hydrogen and exhaust-only water vapour are emerging to challenge electric vehicles as the world's transportation of the future.
At motor shows on two continents three car makers have unveiled hydrogen fuel cell vehicles to be delivered to the public as early as next spring.
South Korea's Hyundai will be the first to the mass market in the US. It unveiled a hydrogen-powered Tucson small SUV at the Los Angeles Auto Show that will be leased to consumers.
Hydrogen cars are appealing because unlike electric vehicles, they have the range of a typical petrol car and can be refuelled quickly. Experts say the industry has also overcome safety and reliability concerns that have hindered distribution in the past.
But hydrogen cars still have a glaring downside - refuelling stations are scarce and costly to build.
Consumers can expect costs in line with some luxury models. In Tokyo, Toyota promised a price of $50,000 to $100,000, and as close to the lower figure as possible. That's comparable to its Lexus luxury sedans, but a range that makes the once space-age experiment with fuel cells more credible.
Hyundai says it will lease the Tucsons for 499 dollars (£312) a month for three years with 3,000 dollars (£1,800) down - and it is offering to pay the hydrogen and maintenance costs.
The company will start leasing in the Los Angeles area, where most of the state's nine fuelling stations are located. California has allocated 100 million dollars (£62.5m) to build 100 more stations. Japan's Honda would not reveal any pricing details.
Even as battery-powered and hybrid-electric cars took on conventional petrol models in the past decade, car makers continued research into hydrogen fuel cells, said Paul Mutolo, director of external partnerships for the Cornell University Energy Materials Centre. Manufacturers are now limited only by costs and the lack of filling stations, he said.
Hydrogen cars, Mr Mutolo said, have an advantage over battery-powered electric cars because drivers do not have to worry about running out of electricity and having to wait hours for recharging.
"It's very similar to the kind of behaviour that drivers have come to expect from their gasoline cars," he said.
Hydrogen fuel cells use a complex chemical process to separate electrons and protons in hydrogen gas molecules. The electrons move toward a positive pole and the movement creates electricity. That powers a car's electric motor, which turns the wheels.
Since the hydrogen is not burned, there is no pollution. Instead, oxygen is also pumped into the system and when it meets the hydrogen ions and electrons, it creates water and heat. The only by-product is water.
A fuel cell produces only about one volt of electricity, so many are stacked to generate enough juice.
Hydrogen costs as little as three dollars (£1.80) for an amount needed to power a car the same distance as a gallon of gasoline, Mr Mutolo said.
Manufacturers will probably lose money on hydrogen cars at first, but costs will decrease as precious metals are reduced in the fuel cells, he said.
Toyota has said its new fuel cell vehicle will go on sale in Japan in 2015 and within a year later in Europe and the US. Its car is a "concept" model called FCV that looks similar to the Prius gas-electric hybrid.
Honda, which has leased about 24 fuel cell cars since 2005, took the wraps off a futuristic-looking FCEV concept vehicle in Los Angeles. It shows the style of a 300-mile range fuel cell car that will be marketed in the US and Japan in 2015.
Stephen Ellis, manager of fuel cell marketing for Honda, would not say where the vehicle would be marketed in the US, but he expects hydrogen fuelling stations to be abundant first in California, and then north east states.
He predicts it will take five years for the stations to reach significant numbers outside California, and up to 25 years to go nationwide.
Hyundai would not say how many fuel-cell Tucsons it expected to lease. The company believes fuel cells will power the next generation of cars, appealing to affluent, environmentally-conscious customers because affordable battery technology has not advanced enough.
"This is the sort of technology that makes batteries look old-fashioned," said the car maker's North American chief executive John Krafcik.
But sceptics say hydrogen fuelling stations are more expensive than electric car charging stations, partly because electricity is almost everywhere and new and safe ways for producing, storing and transferring hydrogen will be needed.
He said he did not see a mass-market fuel cell as viable before 2020.