Officers of the Hong Kong Customs and Excise Department discovered drugs concealed in an underwater sea chest of an ocean-going vessel, and seized 417 kilograms of a substance believed to be cocaine with an estimated market value of about HK$256 million ($32.9 million).

Two people have been arrested so far in the case, according to Customs officers addressing the media on Tuesday.

The vessel set sail from Brazil, and made stopovers in Singapore, Shanghai and Ningbo before arriving in Hong Kong. The drugs bust marks the first time that Hong Kong Customs officers have discovered drugs that were concealed in a hidden underwater sea chest in the hull of an ocean-going vessel.

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“This drug concealment technique is exceptionally rare. Given that the sea chest lies roughly 11 meters beneath the hull, only professional divers could access the site,” said Lau Yuk-lung, head of the Customs Drug Investigation Bureau.

The vessel measures 333 meters in length and 48 meters in width, with a submerged area equivalent to nearly three standard football fields. With sunlight blocked by the vessel’s hull, the underwater search team first deployed an underwater robot to conduct a large-scale sweep. The robot quickly detected several large suspicious packages wrapped in waterproof materials at the 11-meter depth of the sea chest. Customs divers then carried out a more thorough search and found the suspected cocaine.

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Lo Chin-man, group head (Marine Enforcement) of the Ports and Maritime Command of the Customs, said that the deployment of robotic devices significantly reduced the physical efforts needed by the Customs divers while boosting the effectiveness of the search.

The operation was jointly launched by Hong Kong Customs, the Chinese mainland’s Customs anti-smuggling departments, the Hong Kong Police Force and the Fire Services Department. Hong Kong Customs said that the operation has dismantled a large-scale maritime drug supply chain, inflicting a heavy blow on the trafficking ring and curbing drug-related activities at their source.

Fu Yinglin contributed to the story.