Falling in tandem with plunging regional markets, the Sensex at the Bombay Stock Exchange tumbled 512 points, to slip below the key 17,000 mark, at 16,793–its lowest levels in nearly 14 months–in opening trade this morning. The Sensex fell on across-the-board selling by investors, following the downgrade of US government debt by global rating agency, Standard & Poor's. But an encouraging opening in the European markets, and a partial recovery in the regional bourses later helped the Sensex stage a strong recovery, so that it stood a much more modest 122 points, or 0.7 percent in negative territory, at 17,183, in afternoon trade, a short while ago.And after a partial pullback, Asian markets in Japan, China, South Korea, Singapore and Hong Kong were down between 2.2 percent and 3.8 percent, today, following the historic US credit rating downgrade.