Speculators are again piling into an exchange-traded note that lets them bet on how long days of calm will last in the U.S. stock market.

The trade, using a 6-year-old security known as VXX that sees more average daily volume than shares of Microsoft Corp. and Facebook Inc., has usually backfired amid the biggest bull market since the 1990s. That hasn’t curbed its popularity, with the iPath S&P 500 VIX Short-Term Futures ETN poised for an eighth straight week of inflows, a streak not seen in three years. Shares outstanding in the note are at an all-time high.