More than 13 tonnes of hashish sunk to the bottom of the Mediterranean after smugglers carrying the potent stash were intercepted by boats working for Triton, the EU's coastal patrol operation said on Friday.
Triton, charged with looking out for people-traffickers and their human cargoes, spotted the suspicious 25-metre fishing boat on Thursday.
Officers from the Spanish Civil Guard and Italy's Financial Police tailed it and boarded it about 100 nautical miles (185km) to the south of Sardinia, arresting nine Egyptians.
The officers were able to count 370 packets of hashish containing just under 15 tonnes of the powerful form of cannabis before they realised that the boat was holed in its engine room and about to sink.
A combination of its abrupt halt and its heavy load ensured it started taking on water rapidly and it soon went under in waters 2,600 metres (8,500 feet) deep.
"There was only time to recover about 1,600 kilogrammes (1.6 tonnes, 3,500 pounds) of the hashish," the officer in charge of the operation, Colonel Antonino Iraso, told AFP.
Triton is a multinational operation run by the European coastal agency Frontex and is charged with patrolling waters off the bloc's southern shores, particularly around Italy.
It is supposed to replace "Mare Nostrum" the Italian navy's search-and-rescue mission which has, since October 2013, picked up more than 160,000 migrants from mostly distressed boats operated by traffickers.