For Your Eyes Only

The British spy boat St Georges, which holds the Automatic Targeting Attack Communicator (ATAC), the system used by the Ministry of Defence to communicate with and co-ordinate the Royal Navy's fleet of Polaris submarines, is sunk after accidentally trawling an old naval mine in the Ionian Sea. MI6 agent James Bond is ordered by the Minister of Defence, Sir Frederick Gray and MI6 Chief of Staff, Bill Tanner, to retrieve the ATAC before the Soviets, as the transmitter could order attacks by the submarines' Polaris ballistic missiles.
The head of the KGB, General Gogol has also learnt of the fate of the St Georges and already notified his contact in Greece. A marine archaeologist, Sir Timothy Havelock, who had been asked by the British to secretly locate the St Georges, is murdered with his wife by a Cuban hitman, Hector Gonzales. Bond goes to Spain to find out who hired Gonzales.
While spying on Gonzales' villa, Bond is captured by his men, but manages to escape as Gonzales is killed by an arrow. Outside, he finds the assassin was Melina Havelock, the daughter of Sir Timothy, and the two escape. With the help of Q, Bond identifies the man he saw paying off Gonzales as Emile Leopold Locque, and then goes to Locque's possible base in Italy. There Bond meets his contact, Luigi Ferrara, and a well-connected Greek businessman and intelligence informant, Aris Kristatos, who tells Bond that Locque is employed by Milos Columbo, known as "the Dove" in the Greek underworld, Kristatos' former resistance partner during the Second World War. After Bond goes with Kristatos' protÃ©gÃ©e, figure skater Bibi Dahl, to a biathlon course, a group of three men, which includes East German biathlete Eric Kriegler, chases Bond, trying to kill him. Bond escapes, and then goes with Ferrara to bid Bibi farewell in an ice rink, where he fends off another attempt on his life by men in ice hockey gear. Ferrara is killed in his car, with a dove pin in his hand. Bond then travels to Corfu in pursuit of Columbo.
There, at the casino, Bond meets with Kristatos and asks how to meet Columbo, not knowing that Columbo's men are secretly recording their conversation. After Columbo and his mistress, Countess Lisl von Schlaf, argue, Bond offers to escort her home with Kristatos' car and driver. The two then spend the night together. In the morning Lisl and Bond are ambushed by Locque and Lisl is killed. Bond is captured by Columbo's men before Locque can kill him; Columbo then tells Bond that Locque was actually hired by Kristatos, who is working for the KGB to retrieve the ATAC. Bond accompanies Columbo and his crew on a raid on one of Kristatos' opium-processing warehouses in Albania, where Bond uncovers naval mines similar to the one that sank the St Georges, suggesting it was not an accident. After the base is destroyed, Bond chases Locque and kills him.
Afterwards, Bond meets with Melina, and they recover the ATAC from the wreckage of the St Georges, but Kristatos is waiting for them when they surface and he takes the ATAC. After the two escape an assassination attempt, they discover Kristatos' rendezvous point when Melina's parrot repeats the phrase "ATAC to St Cyril's". With the help of Columbo and his men, Bond and Melina break into St Cyril's, an abandoned mountaintop monastery. As Columbo confronts Kristatos, Bond kills the biathlete Kriegler.
Bond retrieves the ATAC system and stops Melina from killing Kristatos after he surrenders. Kristatos tries to kill Bond with a hidden flick knife, but is killed by a knife thrown by Columbo; Gogol arrives by helicopter to collect the ATAC, but Bond destroys it first. Bond and Melina later spend a romantic evening aboard her father's yacht.

Question: what is sunk after accidentally trawling an old naval mine in the Ionian Sea?
Answer: Polaris submarines; Polaris submarines