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The journalist stands by a train and pours himself a glass of liquid from the exhaust pipe.

"You can't smell it, you can't taste it", he says. "That's because it's water. Pure water."

This train is powered by hydrogen. It doesn't spit out harmful, polluting fumes - just heat and pure H20.

Hydrogen has been used as a power source since the 1930s, when it filled airshipsAlso known as Zeppelins, these passenger aircraft are kept afloat by a giant balloon filled with a gas - such as hydrogen or helium - that is lighter than air . making transatlantic crossings. More recently, hydrogen fuel has powered NASA's rockets.

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Now, experts are using hydrogen cells in large vehicles to replace electric batteries, which are extremely heavy and take hours to charge.

Hydrogen is lighter than air, and fuel cells take minutes to refill. As it is also the most common elementA pure substance that cannot be broken down by chemistry. There are more than 100 elements, and they make up the periodic table. in the universe, supplies of it are unlimited.

But producing hydrogen cells is riskyAs hydrogen is highly explosive, very careful handling is needed to avoid danger. and expensive. The gas is so light it must be compressed to 300 times more pressure than a car tyre.

Elon MuskA South African-born entrepreneur whose companies have included the online payment service PayPal. has called hydrogen cells "fool cells". But experts say the 2020s will be the "decade of hydrogen".

Of course, it is! Hydrogen is already powering trains and buses. Some companies are now using hydrogen to run charging stations for electric cars. Unlike coal and oil, it is a clean fuel that doesn't pollute the air or produce CO2. Best of all, hydrogen is the most common element in the universe, so we have unlimited access to it.

It's not that simple. First, there are very few hydrogen charging stations, with only 11 in the UK. Second, there may be plenty of hydrogen in the universe, but it doesn't occur naturally - it has to be chemically removedScientists use a chemical reaction to break water molecules down into hydrogen and oxygen. from water. At the moment, the main way to do this is to use fossil fuels. So, while hydrogen is clean, making it is not.