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This means, they say, that the nodules sitting on the seabed could generate electric currents large enough to split, or electrolyse, molecules of seawater.

So for hydrogen to be considered "green", electricity from renewable sources has to be used to electrolyse water.

"What we're envisioning is to use electricity from wind and solar out at sea, to boil and electrolyse seawater to produce green hydrogen," Doh Deog-hee, President of Korea Maritime and Ocean University said.

"We use electricity to electrolyse water and generate hydrogen. And then we're using air to add nitrogen to the system," explains Behzad Partoon, a postdoctoral researcher from Aarhus University.

The basic idea, as is done here, of using sunlight through solar PV to electrolyse water, store the hydrogen then, when the power is required, reverse the process.