Oil traders are keeping record amounts of fuel in independent storage tanks in Amsterdam, Rotterdam and Antwerp, the continent’s oil-shipment hub, according to a firm that monitors the terminals.

A total of 5.799 million metric tons of gasoline, diesel and other processed fuels are being stored, according to data on Thursday from PJK International. That’s the most since the firm began collating the information in 1995, Pieter Kulsen, the Breda, Netherlands-based firm’s founder said by phone.

A 48 percent plunge in Brent crude in 2014 helped create what’s called a contango oil-price structure, where immediate fuel prices are cheaper than those in later months. Gasoline and gasoil, a product near identical to diesel, both cost about $30 more in December than they do in February, according to data from exchanges compiled by Bloomberg. Such a gap can create an financial incentive to store.

“Those two products have been quite substantially higher,” Kulsen said. “The contango swap curve is stimulating storage: it’s profitable to store product.”

The 3 million tons of gasoil being stored are the highest since September 2009 after increasing for the last three weeks. The 1.047 million tons of gasoline are the most since April last year.