Peil confirmed that a small fracture at the base of Schumacher's skull had caused the problem and led to his decision on Tuesday to call off the comeback.

CNN: Schumacher's turmoil over failed F1 comeback

He underwent a routine arthroscopy and micro-fracture to two small areas in his right knee and will be non weight-bearing for six to eight weeks.

BBC: Flintoff undergoes knee surgery

The process involves pumping water containing small percentages of other materials into the porous shale under pressure to hydraulically fracture it and release the trapped oil and gas.

FORBES: Promised Land's Fracking Fictions: OPEC Goes Hollywood With Crocudrama

"Hydraulic fracturing in a well for shale gas development, which involves injection of fluids to fracture the shale and release the gas up the well, has been confirmed as the cause for small felt seismic events at one location in the world, " the report states.

CNN: Debate over fracking, quakes gets louder