World stock markets have been in meltdown from Shanghai and Shenzhen to London and New York. A sea of red blighted the computer screens of the stock exchanges everywhere in a panic global sell-off. Shock and disbelief among investors was ubiquitous. Even as the Dow Jones claws its way back from its worst losses, extreme volatility pervades the entire system. Could this crash be a one-off event, which will quickly return to normal, or the beginning of a series of shocks in an unstoppable chain of events?