Falling for the third straight session, the Sensex at the Bombay Stock Exchange tanked 546 points-its biggest single day fall in 18 months- to close well below the key 18,000 mark, at 17,632, today, on intense all-round selling pressure. The markets fell on macro-economic concerns about oil prices and inflation, the political crisis in Libya, and on weak Asian and European bourses. The Nifty at the National Stock Exchange plummeted 175 points, or 3.2 percent, to 5,263. Stock markets in Japan, Hong Kong, South Korea and Singapore lost between 0.5 percent and 1.3 percent.