These are boxes containing air that is super-saturated with water vapour.

ECONOMIST: Global warming

Hydrogen has a high energy density and is clean - burning to leave only water vapour.

BBC: Hydrogen's energy promise improves

Finally, Declan looked at the BMW Hydrogen 7 - its only emission is water vapour but the technology for this type of green car is still being developed and currently, there are no models on sale.

BBC: NEWS | Programmes | Breakfast | Low carbon family: "Green" cars

He thus put together a collection of apparatus that would not have disgraced the set of a mad-scientist horror film, and filled it with a mixture of methane, ammonia, hydrogen and water vapour that matched contemporary ideas of what the primitive terrestrial atmosphere had been composed of.

ECONOMIST: Stanley Miller