The S&P 500 lost more than 50% from its 2007 peak during the global financial crisis.
The S&P 500 reached a closing high of 1,565.15 on October 9, 2007. It later fell to a closing low of 676.53 on March 9, 2009, a decline of roughly 56.8% from that peak. The fall reflected the collapse of housing-related credit, failures and rescues in financial institutions, and a deep recession.
Because the S&P 500 includes 500 large U.S. companies, it is often used as a broad gauge of American equities. Its decline was therefore more representative of the overall market than the movement of a single company or narrow sector, although it remained a U.S. index rather than a global one.
The Dow also suffered a major fall, and the Nasdaq was affected especially strongly in technology-related periods. A common error is to cite the intraday low when the question asks about closing values; those measures are different.