just one year after being commissioned. The subsequent investigation determined that the explosion was probably accidental, though sabotage could not be completely ruled out. The event shook Russian public opinion. The Russians continued work on two additional dreadnoughts under construction, and the balance of power remained in Russian hands until the collapse of Russian resistance in November 1917. To support the Anglo-French attack on the Dardanelles, British, French and Australian submarines were sent into the Black Sea in the spring of 1915. A number of Turkish supply ships and warships were sunk, while several submarines were lost. The boats were withdrawn at the evacuation of the Dardanelles in January 1916. The small Romanian Black Sea Fleet defended the port of Sulina throughout the second half of 1916, causing the sinking of one German submarine. Its minelayer also defended the Danube Delta from inland, leading to the sinking of one Austro-Hungarian Danube monitor. (See also Romanian Black Sea Fleet during World War I) Despite losing most of their coastline to the Central Powers after the Second Battle of Cobadin in October 1916, the Romanians still managed to keep the mouths of the Danube and the Danube Delta under their control, due to the combined actions of their riverine flotilla of four monitors and the protected cruiser Elisabeta, based at Sulina. The Romanian Navy repelled two attacks of the Imperial German Navy on the port of Sulina. The first attack took place on 30 September 1916, when the Romanian torpedo boat Smeul engaged the German submarine UB-42 near Sulina, damaging her periscope and conning tower and forcing her to retreat. The second attack took place on 7 November, when German Friedrichshafen FF.33 seaplanes bombarded Sulina but two of them were shot down into the sea by Romanian anti-aircraft defenses (including the cruiser Elisabeta) and were subsequently captured by Romanian motorboats. In mid-November 1916, UC-15, the only minelaying submarine of the Central Powers in the Black Sea, was sent to lay 12 mines off Sulina and never returned, being most likely sunk by her own mines along with all of her crew. She could have also been sunk by the barrage of 30 mines laid at Sulina by the Romanian minelayer Alexandru cel Bun. When Bulgaria entered World War I in 1915, its navy consisted mainly of a French-built torpedo gunboat called Nadezhda and six torpedo boats. It mostly engaged in mine warfare