The Bombay Stock Exchange benchmark Sensex was down by nearly 170 points in the first 45 minutes of the trading today on emergence of profit booking. Sensex, which opened nearly 110 points high, failed to keep up the momentum and dipped below 18,000 levels by slipping to 17,980 points at 1000 hrs today.In a similar manner, the wide-based National Stock Exchange's Nifty was also trading nearly 38 points lower at 5,408 at the same time.Stock brokers said, profit-booking of the gains made by blue chips at the outset of the trading session, dragged the Sensex down. In particular, banking, realty, IT and oil and gas stocks were under selling pressure.