U.S. stocks plunged Monday, pushing the Standard & Poor's 500 index to its lowest level of the year. The Dow Jones industrial average fell 507.53, or 2.1 percent, to 23,592. The broader S&P 500 index fell 54.01, or 2.1 percent, to 2,545.94. Earlier, the index was down as much as 2.5 percent. The technology-heavy Nasdaq composite index fell 156.93, or 2.3 percent, to 6,753.73.