When was the closest the United States came to an economic collapse?  September 17, 2008, as investors withdrew a record $144.5 billion from money market accounts, usually considered the safest of investments. That's where companies, sovereign wealth funds, and even retirees keep their cash. During a normal week, only about $7 billion is withdrawn.

Panicked investors were moving the funds to U.S. Treasuries, causing yields to drop below zero.

In other words, investors were so panicked that they no longer cared if they got any return on their investment...they just didn't want to lose capital.

Money market funds are where businesses keep their cash overnight to use for day-to-day operations. If those funds had run dry, your grocery store shelves would have gone empty within weeks. 

As described in the Wall Street Journal: Huddled in his office Wednesday with top advisers, Treasury SecretaryHenry Paulson watched his financial-data terminal with alarm as one market after another began go haywire. Investors were fleeing money-market mutual funds, long considered ultra-safe. The market froze for the short-term loans that banks rely on to fund their day-to-day business. Without such mechanisms, the economy would grind to a halt. Companies would be unable to fund their daily operations. Soon, consumers would panic.

What caused this unprecedented run on supposedly safe money markets? The $62.6 billion Reserve Primary Fund "broke the buck" the day before.

That means investors were taking out so much that it couldn't keep its share price at the one dollar value all money markets use as a benchmark. They were worried that the Fund had no cash left because of its investments in now-bankrupt Lehman Brothers. 

Banks were also hoarding cash, too panicked to lend to each other or purchase any assets, for fear of taking on bad debt. Normally, financial institutions have about $2 billion on hand at any given time. By Thursday of that week, they had an unprecedented $190 billion, to prepare for further redemptions. In other words, the economy was on the precipice of a full-scale run on the banks - and not by worried depositors as in the 1930s, but by corporate investors.

Without these funds' participation, the $1.7 trillion commercial-paper market, which finances automakers' lending arms or banks credit-card units, faced higher costs. Without commercial paper, "factories would have to shut down, people would lose their jobs and there would be an effect on the real economy," says Paul Schott Stevens, president of the Investment Company Institute mutual-fund trade group.

Treasury Secretary Henry Paulson conferred with Federal Reserve Chairman Ben Bernanke, who agreed that the problem was beyond the scope of monetary policy. The Federal government was the only entity large enough to step in and stop the madness. The money market run triggered the bank bailout bill. Congress approved the bill to pay as much as $700 billion to bail out investment banks who purchased mortgage-backed securities that were in danger of defaulting. The money market run showed just how close the global economy was to a catastrophic meltdown. When Congress asked Paulson what would happen if the bailout weren't approved, he quietly replied, "Heaven help us all." (Source: WSJ, Shock Forces Paulson's Hand, September 20, 2008)