UNG is an (infamous) ETF that tracks natural gas futures. Unfortunately for UNG, natural gas has been in one of the biggest bear market since 2008. Moreover, UNG invests in the front-month future contract that continuously needs to be rolled to the next month. If the futures are in a state of "contango" this will eat into the returns on the ETF. Here is the chart of UNG which shows a recent break out to new lows.
Commodities tend to be trending assets, so this could be a good time to reenter your short. Contango for natural gas futures appears to be high and fundamentals appear to be awful according to this WSJ article due to horizontal fracking. Despite this supply it is very difficult to transport natural gas outside of the US unlike crude oil. Basically, UNG is a falling knife from what I can tell. Although as usual with trading - caveat emptor.
Disclosure: Short UNG