A sell-off in the global stock markets led to the Sensex at the Bombay Stock Exchange plunging 300 points, or 1.8 per cent, to slip below the 17,000 mark, at 16,766 in opening trade, on Thursday. Later, the sell-off worsened, and the Sensex had lost a massive 537 points, or 3.2 percent, to 16,528, in afternoon trade, a short while ago.Stock markets across the globe fell sharply, on panic selling by investors, after the US Federal Reserve, or central bank, warned that the outlook for the US economy remains grim. So the US Dow Jones Industrial Average had tumbled 2.5 per cent, overnight. Today, Asian stocks markets plummeted, with the key stock indices in Japan, China, Hong Kong, Indonesia, Taiwan, Singapore and South Korea being hammered down by between 2 percent and 6 percent. And the European markets in the UK, Germany and France had been beaten down by between 3 percent and 3.5 percent in initial trade, today.