iPath S&P 500 VIX Short-Term Futures ETN (VXX)

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Due to contango, this ETN will go to ZERO over the long term. It (as well the 2X ETN UVXY) is the closest thing to a guaranteed short I have ever seen. The only way to make money long VXX is to get the timing exactly right for a market crash, but that's almost impossible. Even if you do get it right, your gains will be short lived since volatility always calms down, and contango erodes the value of the ETN.