An Olympic torch relay will run for 70 days ahead of the London 2012 Olympic Games. So what is this relay, and why all the fuss?

"It is an utterly thrilling thing to do," says Philip Barker.

The Olympic historian and author has a lump in the throat just at the memory of running, torch clasped in his hand, high in the Taygetos mountains above Sparta, Greece.

He was a part of the torch relay, the human-powered running feat that bore the flame on its journey from its source, Olympia, to the Atlanta Games in 1996....