Sensex tumbled 179 points, while Nifty fell 76.15 points or 0.48 per cent to 15,691.40 on Thursday, weighed down by losses in index heavyweights and a largely negative trend in global equities, even as new coronavirus infection and death numbers kept falling, with active cases down to a 71-day low at 8,26,740, according to a Union health ministry release.