More than 15 tons of marijuana hidden in a mattress shipment was seized at the Otay Mesa cargo port of entry — the largest narcotics bust in the history of that border crossing and the second largest seizure at any crossing nationwide.

U.S. Customs and Border Protection officers made the find as they inspected a truck claiming to carry mattresses and cushions on Thursday evening.

The officer inspecting it referred it to an x-ray exam, where an anomaly was detected.

As the truck was docked for a more intense examination, a CBP officer raised the trailer's door and found plastic-wrapped packages, stacked floor to ceiling and front to back, filling nearly the entire truck. Just a few mattresses were stacked along the wall at the other end.

Officers tallied the find, which came to 1,296 packages containing about 31,598 pounds of marijuana. That load has an estimated street value of about $18.96 million.

The driver, a 46-year-old Mexican citizen with a valid border crossing card, was turned over to the U.S. Immigration and Customs Enforcement Homeland Security Investigations. The tractor-trailer, cargo shipment and drugs were all seized by CBP officers.

The Otay Mesa cargo port of entry saw its last record-breaking bust in 2003 when 19,999 pounds of marijuana was discovered. The largest seizure ever recorded by CBP officers was 35,265 pounds of marijuana at the Calexico East port of entry in 2013.