Benchmark indices Sensex and Nifty closed flat on Monday in a highly volatile trade amid a weak trend in global markets and continuous foreign fund outflows. After gyrating 461.6 points during the day, the 30-share BSE Sensex eked out a marginal gain of 14.54 points or 0.02 per cent to settle at 66,023.69. During the day, it hit a high of 66,225.63 and a low of 65,764.03. The broader Nifty settled marginally up 0.30 points at 19,674.55.