Though crude rallied 5 percent on Wednesday, some traders are expecting more downside ahead.

The price of oil has fallen to half of what it was last summer, and the ETF that seeks to track oil prices (trading under the ticker symbol USO) has plunged along with it. With the most recent short-term pop, a trader placed a large bet that oil will instead see further downside in the next six weeks.

On Wednesday, when options volume was around 50 percent higher than average, one trader bought 66,000 of a 16.50/14.50 May put spread on the USO priced at 50 cents. Since each contract controls 100 shares, the trader risked $3.3 million.

The trade breaks even should the USO decline to $16 per share. However, if the USO sinks to as low as $14.50—or 17.5 percent below Wednesday's closing price—by May 15, the trade could make back nearly $10 million or triple the amount wagered.