While searching for targets in the vicinity of Venice in August 1915, U-12 struck a mine that blew her stern off, and sank with all hands, becoming the first Austro-Hungarian submarine sunk in the war.

[4] According to one source, SS-3 was initially propelled by a pair of electric motors for surface running,[7] but had them replaced with twin 6-cylinder gasoline engines of 300 brake horsepower (220 kW) each when they proved disappointing during trials.

[8][Note 2] It is not specifically reported for U-12, but the other U-5-class boats both suffered from inadequate ventilation, which resulted in frequent intoxication of the crew from the engine exhaust.

[3] U-12's activities over the early part of the war are not reported, but the boat's armament was augmented by a 3.7 cm (1.5 in)/23 quick-firing (QF) deck gun in November 1914.

[1] On 21 December 1914, Lerch and U-12 chanced upon the French dreadnought Jean Bart in the Straits of Otranto steaming at a leisurely 9 knots (17 km/h; 10 mph) and unprotected by escort ships.

[11] U-12 hit French Admiral Lapeyrère's flagship with a single torpedo in the bow,[12] destroying the battleship's wine storeroom but sparing her forward magazine.

[13] Jean Bart's watertight compartments saved the ship,[12] which made her way to Malta to undergo repairs at the British dockyards there.

Two days later, when Italian workers were dredging to try to determine the object Rosolino Pilo had hit, they heard a heavy explosion.