Customs officers said on Tuesday that they intercepted 417 kilogrammes of suspected cocaine that was hidden in a ship's underwater sea chest in the first such smuggling method to be detected in the city.

The haul, with an estimated street value of HK$256 million, led to the arrest of two suspects.

According to Lau Yuk-lung, head of the customs drug investigation bureau, authorities had shared information with mainland counterparts indicating a drug trafficking group might be using large cargo vessels to move narcotics.

While drugs had previously been hidden inside containers or other concealed areas on ships, he said this case revealed drug traffickers were adopting a new tactic.

After extensive analyses of shipping routes, vessel structures and intelligence, Lau said, officers narrowed down their targets to a massive cargo ship, measuring around 333 metres long and 48 metres wide, which arrived from Brazil via Singapore, Shanghai and Ningbo.

"We immediately launched 24-hour surveillance of this ship. Finally, on November 4, 2025, the cargo ship entered Hong Kong waters," he said.

"We promptly dispatched customs vessels to escort the cargo ship to our designated location for clearance and inspection."

Lau said a thorough search led officers to the underwater sea chest – a below-the-waterline compartment built into the hull that is used to draw seawater for engine cooling and other functions.

Located about 11 metres below deck with a narrow entrance, he said the concealed chamber contained a large quantity of suspected cocaine wrapped in waterproof material.

Lau said officers believe the criminal group involved intended to use the large ship as a floating storage warehouse to distribute drugs across regions.

The intact packaging, he said, suggests the entire batch was intercepted before any distribution could take place, preventing a significant quantity of drugs from reaching the market.