In January 2004 three special new buses appeared on the streets of London.
They run on hydrogen and are pollution free, but they cost about '1 million each. They are part of a European scheme to try out hydrogen fuel cells as an alternative to petrol and diesel engines.
Hydrogen can be transported and turned into electricity on the bus, in a device called a fuel cell. But transporting the hydrogen to power the cells can be tricky.
Each of the London buses has a canister of hydrogen gas on its roof. This can carry enough gas to drive for 200 kilometres.
Hydrogen vehicles are very clean, producing only water as waste.
But it doesn't really matter that the buses are pollution-free if we have to pump out lots of greenhouse gases in order to make the hydrogen fuel that they use.
The best way to make hydrogen uses water and renewable electricity, but in the short term it is more likely that we will make the hydrogen for our fuel cells from natural gas.
This still produces carbon dioxide, but fortunately it is less than the amount that the diesel engine from an ordinary bus would pump into the atmosphere.
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Fascinating fact
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Hydrogen bus
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Hydrogen cannisters
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Watch: Fab fuel cells
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