The S&P 500 lost approximately 57% from its October 2007 peak to its March 2009 low during the global financial crisis.
The index closed at 1,565.15 on October 9, 2007, before the housing and credit crisis fully spread through the financial system. It later closed at 676.53 on March 9, 2009, after a prolonged worldwide sell-off.
The decline reflected failures and rescues involving major financial institutions, collapsing confidence in mortgage-linked securities, restricted credit, and the deepening recession. Stock markets outside the United States also experienced severe losses.
The 57% figure describes the S&P 500's price decline between those closing levels; it is not the same as the loss for every investor or every market. Dividends, investment dates, currencies, and different indexes would produce different results.