Arrows show imbalances of debt exposure between borrowers in one country and banks in another; arrows point from debtors to their bank creditors. Arrow widths are proportional to the balance of money owed. For example, French borrowers owe Italian banks $50.6 billion; Italian borrowers owe French banks $416.4 billion. The difference — their imbalance — shows France's banking system more exposed to Italian debtors by about $365.8 billion.

The risk to countries’ debts and economies is indicated by color: More worrisome

Greece amassed a huge debt that it has scant hope of repaying. A chaotic Greek default could hurt all European banks and pension funds that have extended Greece credit and cause a wider bank panic. A financial firewall might halt contagion by backstopping the credit of four other shaky nations — Ireland, Portugal, Spain and Italy.

If there is no firewall or if it is inadequate, it would be easy to imagine a run on banks. The euro zone’s single currency makes it easy to shift money across borders from risky economies to safer ones. That and the lack of central banks in each country -- those went away in 1999 with the arrival of the euro — make the euro zone “the ultimate contagion machine,” says Kenneth Rogoff, a Harvard economist.

Euro Zone

If no preventative measures are taken, a chain of events like this could unfold: In reaction to a Greek collapse, investors become worried about their exposure to other risks in the region. Borrowing costs rise for Ireland, Italy, Portugal and Spain, adding to their debt loads.

Italy may not be able to protect its banks if there is a loss of confidence. French banks, burdened with all manner of Italian debt, could totter. Money could flee to safer countries like Germany in a matter of hours.