The problem is not merely that holders of Greek government debt would dump their investments, or even that they would ditch their Spanish and Portuguese bonds while they were at it. It is that government debt is the very bedrock of the financial system: should Greek government bonds collapse, the country's banking system would become insolvent overnight. In fact, banks throughout the euro area would be at risk, given that they tend to hold so much of their neighbours' government debt. That, at least, is the theory, but as was the case in the aftermath of Lehman's collapse, no one really knows how great their exposure is.