Yale geologist Sean McMahon, John Parnell of the University of Aberdeen's School of Geosciences, and Nigel J.F. Blamey at Brock University investigated sources of hydrogen in the Earth's subsurface, by analyzing rocks in Sudbury, Scotland and South Africa. The geologists found that friction in earthquakes produces hydrogen--tipped off by earlier studies describing experiments that predicted hydrogen production during earthquakes, according to McMahon. They furthered this work to see if rocks from faults retained any of this extra hydrogen. They were surprisingly hydrogen-rich, and when they rubbed during an event like an earthquake or meteorite collision, the hydrogen escaped from pockets that had formed in the rocks.