(Bloomberg) -- Less than two hours before U.S. Federal Reserve Chair Janet Yellen took center stage this week, an obscure ETF managed by Pimco was the site of aggressive speculation on the rally in Treasuries.

Just before 1 p.m. in New York on Wednesday, the Pimco 25+ Year Zero Coupon U.S. Treasury Index ETF saw a $138 million block trade, representing more than 60% of the outstanding shares, according to data compiled by Bloomberg. It was a 30-fold increase over the average daily trading volume in a fund that’s structured to let investors wager on an asset class that does best when rates are falling.

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