iPath Dow Jones UBS Natural Gas Total Return Subindex ETN ( GAZ) -3.9% Natural gas futures are tumbling for a second day, pushing GAZ and the United States Natural Gas Fund ( UNG) to notable declines. UNG continues to flirt with all-time lows. This is not a fund for the faint of heart. At the same time that ETFs linked to the commodity are tumbling, the equity-tracking First Trust ISE Revere Natural Gas Index Fund ( FCG) is trading in relatively unchanged territory. iPath S&P 500 VIX Short Term Futures ETN ( VXX) -2.6% The market's strong performance is doing little to help VXX as it continues to push lower. The ETN has failed to secure a single day of gains during the opening six trading sessions of 2012. This sell-off has pushed it to levels last seen in early August. The fear-tracking VIX index is struggling as well. Now that it has pierced through the lows seen during the final days of 2011, it will be interesting to see if it has further to fall. All prices were as of 2:14 PM EST. Written by Don Dion in Williamstown, Mass.