Is our National Debt like a ticking time-bomb? If so, what will cause the bomb to detonate, and what happens when it does?

http://www.cracked.com/funny-4136-national-debt/

Unbelieveable that it’s been 172 years since our national debt was zero.

I know that one force or another will have to deal with this debt, whether it’s by repaying it, or another solution.