Speaking of US debt, it's actually well North of $60 Trillion Dollars some studies are saying as high as $70 Trillion Dollars.
The problem is when you look at sources like the US debt clock which stands at around $18 Trillion what you actually see is only the US Governments "reported" debt not the unreported debt. What the reported debt doesn't show you are current government obligations like Medicade, Social Security obligations which by the way is running close to $25 Trillion in debt itself, deposit insurance, loan guarantees to name a few.
If you want to get your head around "Off Balance Sheet Liabilities" here are a few sources to absorb:
Off-Balance-Sheet Federal Liabilities
http://www.nber.org/papers/w19253US debt six times greater than declared - study
http://rt.com/usa/us-debt-study-hamilton-economy-103/US debt six times higher than declared
http://www.presstv.com/detail/2013/08/07/317608/us-debt-six-times-higher-than-declared/