Ashes-holders England, already 1-0 up in the five-match series after a 14-run win at Trent Bridge last week, were 333 for five at stumps on yesterday's third day with Root 178 not out.
Root's second Test century was currently worth exactly 50 more than Australia managed in their meagre first innings 128.
England now had a colossal lead of 566 with their arch-rivals already needing to make far more than the Test record fourth innings victory total of 418 for seven by the West Indies against Australia at St John's in 2002/03 if they were somehow to square the series at 1-1.
"It was a horror day for Australia, it was probably even worse than Friday," McGrath, a member of BBC Radio's Test Match Special commentary team, said.
"England saw them off early on and then the runs starting to tick over as the day wore on."
McGrath was also bemused as to why Australia didn't take the new ball Saturday, albeit England were 172 for four -- a lead of 405 runs - when it became available after 80 overs.
"It became too easy at times. Nothing shy of a miracle can save them (Australia) from here.
