In September 2006, Nouriel Roubini told the International Monetary Fund what it didn’t want to hear. Standing before an audience of economists at the organization’s headquarters, the New York University professor warned that the U.S. housing market would soon collapse — and, quite possibly, bring the global financial system down with it. Real-estate values had been propped up by unsustainably shady lending practices, Roubini explained. Once those prices came back to earth, millions of underwater homeowners would default on their mortgages, trillions of dollars worth of mortgage-backed securities would unravel, and hedge funds, investment banks, and lenders like Fannie Mae and Freddie Mac could sink into insolvency.