from the cited article...

According to Professor Friedemann Freund and colleagues at Nasa's Ames Research Center in California, the {hydrogen} gas is produced when water molecules trapped inside molten rock break down to release hydrogen.

"In the top 20 kilometres of the Earth's crust, the conditions are right to produce a nearly inexhaustible supply of hydrogen," said Professor Freund.

Studies by the team of common rock types such as granite and olivine have revealed extraordinarily high levels of trapped hydrogen. Professor Freund said that his team had "tantalizing evidence" that as much as 1,000 litres of hydrogen may be trapped in each cubic metre of rock.

-p (hey, they're Californians, probably wack jobs?)