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How can all these countries owe so much money?  Why & where did they borrow it from?
Here is an overview of who owns US Debt from Wikipedia, it indicates that approximately 1/3rd of US debt is held by foreigners (mainly the central banks of other countries), approximately 1/2 of US Debt is held by the federal reserve, and the rest is owned by various America organizations (mutual funds, pension funds, etc). The money is loaned via bonds, treasury bills, etc. When you put money in your pension fund, you very likely buying US debt. The US Treasury department all has a comprehensive page about how public debt works in the United States here: an overview of public debt from the treasury. I wasn't able to find a similar breakdown for other countries, but Wikipedia has a comprehensive list of how much debt is owed by other countries: a list of countries by public debt.