The Great Recession was a recession observed in national economies globally during the late 2000s. The International Monetary Fund (IMF) concluded that it was the most severe economic meltdown since the Great Depression. The causes of the Great Recession include vulnerabilities that developed in the financial system, along with a series of triggering events that began with the bursting of the United States housing bubble in 2005–2006. When housing prices fell and homeowners began to walk away from their mortgages, the value of mortgage-backed securities held by investment banks declined, causing several to collapse or be bailed out. This 2007–2008 phase was called the subprime mortgage crisis. The Great Recession began in the U.S. officially in December 2007 and lasted until June 2009, thus extending over 19 months.