You'd think investors would sour on a bet against bonds that's lost 25 percent in seven months. Turns out some people have a very high tolerance for pain.

Investors have poured $1.1 billion into the ProShares UltraShort 20+ Year Treasury exchange-traded fund (ETF) this year, undeterred by the third-worst performance among all U.S. fixed income ETFs. The fund, which acts as an amplified bearish wager on longer-term U.S. Treasuries, has received $49 million in the past week alone and is the eighth-most popular ETF among its peers in 2014, according to data compiled by Bloomberg on flows.

Some bond skeptics just aren't ready to give up their belief that Treasury yields will rise in the next few months, despite waiting years for that to happen with little to show for it. That ETF has lost about 79 percent since its May 2008 inception, Bloomberg data show.