LONDON - The world's greatest athletes descend on London in July for the 2012 Olympics. Reuters' Martyn Herman highlights 10 to watch when the Games get underway:

Usain Bolt (Athletics)

The face of the Beijing Olympics where the Jamaican sprinter's jaw-dropping speed earned him three golds in the 100m, 200m and 4x100m relay and three world records.

He has been pretty much unbeatable when it matters since, winning the five golds from the last two world championships - his only blemish coming in Daegu where he was disqualified after false-starting in the 100m.

Tickets to watch Bolt race in London will be like gold dust, but those with seats better not be late as some predict, if he is firing on all cylinders, he can lower his 100m world record mark below 9.5 seconds.

Usain Bolt of Jamaica celebrates winning the men's 200m final of the athletics competition in the National Stadium at the Beijing 2008 Olympic Games in this Aug 20, 2008 file photograph.[Photo/Agencies]