The mammoth seizure weighed nearly as much as a school bus and the sub was 230 nautical miles from the Azores when it was intercepted.
The semi-submersible eventually sank before authorities could take all its cargo, sending 35 of the 300 packages to the bottom of the Atlantic.
Portuguese police led the operation, assisted by authorities in the UK and US.
The nearly nine-tonne cargo is “the biggest seizure of cocaine ever in Portugal”, a police spokeswoman told AFP news agency.
The boat came from Latin America and had three Colombians and a Venezuelan on board, police said.
“Inside the vessel – 300 bales of cocaine were being transported,” they added.
Such vessels have been dubbed “narco subs”, with cartels using them to try to smuggle drugs undetected.
Sky News’ Alistair Bunkall last year reported on the tactics being used by smugglers off the Iberian peninsula and how, in November, Portuguese security intercepted the kind of narco submarine that is rarely discovered.
Narco subs had previously been used to ferry drugs north from Colombia to central America and Mexico.
But traffickers have been setting their sights further and using them to sail across the Atlantic and Pacific.
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Last March, police said officers had confiscated more than six tonnes of cocaine from a semi-submersible off the Azores while bound for the Iberian peninsula.
The UK’s National Crime Agency played a role in that bust – alongside the US Drug Enforcement Administration and the Spanish Guardia Civil.
In 2023, a sub with two dead bodies and nearly three tonnes of cocaine aboard was seized off the coast of Colombia.
