The BSE benchmark Sensex dipped below the 18,000-points mark after two months at mid-session on Monday, due to brisk selling by funds in heavy-weight stocks, led by banking and metal sectors. Sensex, which opened in a negative zone, rolled down further by 353 points to 17,972 at 1330 hours, a level last seen on March 22. Similarly, the broad-based National Stock Exchange index, Nifty, lost 112 points to 5,374 at the same time.