A German grenade from World War I was found among a shipment of French potatoes which were being imported to Hong Kong for a chip factory.
"The grenade was in an unstable condition because it has been previously discharged but failed to detonate," Superintendent Wilfred Wong Ho-hon told reporters. It was safely detonated after it was discovered at the Calbee snacks factory Saturday.
The police packed the grenade in a drainage channel before blowing it up, Wong said with video of the police detonating it. They also cleared a nearby street and moved cars before they blew it up.
The grenade was eight centimetres (three inches) wide and weighed about one kilogram (two pounds).
"All the information to date suggested that the grenade was imported from France together with the other potatoes," Wong said.
The grenade is believed to have been left in a trench during World War I and accidentally gathered up with potatoes planted a century later in the former battlefield.
"If it was covered in mud, the grenade was likely to have been left behind, dropped by soldiers there during the war, or left there after it was thrown," Hong Kong University military historian Dave Macri said.
"The ditch was then filled up and used as a growing field, and the explosive was tossed into the mix of harvested potatoes... and sent to Hong Kong."
Hong Kong police are used to dealing with old munitions, though more usually US bombs dropped on the city after it fell to the invading Japanese during World War II.
Unexploded wartime bombs or grenades are frequently found by hikers or construction workers in the southern Chinese city, which was the scene of fierce fighting between Japanese and British allied forces in 1941.
The then-British colony was heavily bombed by US and allied forces after the city fell to the Japanese imperial forces.