'BG.5' was an Italian naval attack on major British ships gathered in the Bay of Algeciras, off Gibraltar (17/18 December 1942).

Six frogmen of the Italian 10a Flottiglia MAS, an elite special forces unit, left on three maiale (pig) manned torpedoes via the underwater hatch of the disguised depot ship Olterra, an interned tanker anchored at Algeciras in neutral Spanish waters. The team’s object was an attack on the battleship Nelson and the fleet carriers Furious and Formidable.

The 5,000-ton Olterra was an Italian tanker scuttled by her own crew at Algeciras in the Bay of Gibraltar on 10 June 1940, after the entry of Italy in World War II. She was recovered in 1942 by a special unit of the 10a Flottiglia MAS for use as a clandestine base for from which manned torpedoes, known as maiali (pigs) to the Italians, could be launched for attacks on Allied shipping at Gibraltar. Between 24 September 1940 and 15 September 1942 the Italians attempted six underwater attacks, of which three succeeded in sinking some 40,000 tons of Allied shipping: two of the attacks were made by maiali launched from the submarine Scirè, one by frogmen from the shore, and three by maiali from Olterra. It was during 1940 that Scirè made its first foray into the Bay of Gibraltar, and launched all three maiali: none of the three were successful, though one reached a position only 110 yards (100 m) from the battleship Barham before becoming stuck. The explosion that followed did no damage, but alerted the British to the new threat and led them to organise patrols by boats which dropped small charges whose detonations in the water would create shock waves sufficiently powerful to kill divers.

The submarine re-entered the Bay of Gibraltar in May 1941, and on this occasion its maiali gained better results: they attacked three British merchant vessels, of which the 2,000-ton tanker Fiona Shell was sunk, and the tanker Denby Dale and freighter Durham were damaged.

After Scirè's attacks, Capitano di Fregata Vittorio Moccagatta, commander of the 10a Flottiglia MAS, decided that the limitations of using a submarine as a 'mother ship' for manned torpedoes in Gibraltar meant that it would be more practical to establish a secret base in neutral Spain. A first step in that direction was taken when a member of the flotilla, Antonio Ramognino, rented a bungalow along the coast road near Algeciras, right in front of a bay used an anchorage for Allied convoys. Operations from this Villa Carmela were carried out by combat swimmers, who used limpet mines to sink or damage five merchant vessels between July and September 1942. Olterra was used as an advanced observation post for these missions.

Over the same period another of the flotilla’s officers, Tenente Licio Visintini, a veteran of the previous submarine incursions against Gibraltar, heard about Olterra and conceived the idea of using her as a secret 'mother ship' for maiali. Under the pretext of raising the ship for sale to a Spanish owner, a flotilla team disguised as Italian civilian workers took control of the tanker, which was towed to Algeciras for the start of 'repairs'. The Italian naval personnel were helped by two civilian members of the crew, who had remained on board the partially sunken tanker, together with a Spanish guard, for more than two years to protect the legal rights of the Italian company which owned the ships. Once Olterra was in dock, some of her tanks and a boiler room were modified into a workshop for the assembly and maintenance of maiali. An improvised observation position was also created on the forecastle to keep watch over the Bay of Algeciras and the Allied ships at anchor there. A facade of civilians working to overhaul the ship was established to fool outsiders. The disassembled maiali, spare parts and other equipment were smuggled into Spain by men of the flotilla as supposed materials for the repairs on the ship. Finally, a sliding hatch was opened with a cutting torch below the ship’s waterline, so that the maiali craft could depart and return, in a flooding bilge below the workshop.

By the end of 1942’s autumn, Olterra was ready, the workshop reconstruction complete and all the necessary equipment smuggled into Spain. On 6 December 1942, after taking part in 'Torch', a British naval squadron comprising the battleship Nelson, battle-cruiser Renown, fleet carriers Furious and Formidable, and a number of smaller warships, reached Gibraltar. Visintini planned a mission by three maiali, each of them carrying two divers: the leading torpedo, driven by Visintini himself and Capo Magro, the second by Sottotenente Cella and Sergente Leone, and the third by Guardiamarina Manisco and Capo Varini: Visintini was to attack Nelson, Manisco Formidable and Cella Furious.

The maiali left Olterra during the early hours of 8 December, and at 02.15 the first of the craft reached the boom defences. The motor launches and sentries inside the British base were conscious of the possibility of an attack on the anchored ships. One of the security measures taken by the British after the activities of the Italian combat swimmers during the summer was the deployment to Gibraltar of an underwater bomb disposal unit under the command of Lieutenant Lionel Crabb. A pattern of depth charges was dropped by the motor boats at three-minute intervals. Visintini’s craft was apparently hit by one of the charges and destroyed as the two men were trying to find a gap in the steel net protecting the harbour, and the bodies of two men were recovered by the British some days later and buried at sea.

The second maiale also endured a major British response after by illuminated by a searchlight and, after being chased by anti-submarine boats, its crew decided to scuttle the craft, after which the two men took shelter on a US freighter and discarded their equipment before surrendering to the Gibraltar authorities.

The last maiale was caught in the middle of the general alarm across Gibraltar but nonetheless managed to slip beneath the waters and fool the submarine chasers. Leone, the co-pilot, went missing during the pursuit and was never found, and Cella abandoned his craft thinking that he was still near Gibraltar or, in the best case, stranded close to the Spanish coast. With the idea of becoming a prisoner of war or being arrested and interned by Spanish authorities in mind, Cella surfaced, only to find that he was very close to Olterra. His maiale was recovered by the Italians on the next day.

The two divers captured by the British successfully disinformed their interrogators that the attack had been launched from a submarine, thereby deceiving Allied intelligence. After Visintini’s death, Tenente Ernesto Notari assumed temporary command of the detachment on Olterra, and replacements of men and equipment were despatched from Italy. The British boom defences of the naval anchorage had been improved in the meantime, so the Italians planned their next mission against transport and cargo ships in the anchorage area outside the naval base. Greater impetus was given to the planning by the end of the campaign in North Africa during May 1943, and the imminent launch of an Allied invasion of Italian or Italian-occupied territory on the northern side of the Mediterranean (in fact the 'Husky' invasion of Sicily in July of the same year), which made amphibious warfare ships a priority target. In May 1943 Capitano di Fregata Borghese, of Scirè, assumed command of the 10a Flottiglia MAS, and chose the night of 8 May for the next assault, taking advantage of the bad weather and the phase of the moon.