Think US debt is high? Take a look at Europe's most indebted nations.

In April, the IMF estimated that the US government's debt amounted to 99 percent of gross domestic product. That’s high, but less than four of Europe's five largest debtors.

2. Italy

Italy is the country most worrying European officials today. Earlier this month, they began discussing the possibility that the Italian economy, the fourth-largest in Europe, may end up needing a bailout, too. The cost of insuring its bonds is on the rise, its debt has steadily grown, and its stocks are declining. If Italy falters, it could have a significant impact on the core European economies.

Its sovereign debt ($2.5 trillion, according to the Monitor’s most recent figures) is the largest on the continent and the second-largest relative to GDP: 120 percent in April.

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