 On 27 September 1916 , UB @-@ 7 departed Varna for operations off Sevastopol and was never heard from again . According to some sources , UB @-@ 7 was mined somewhere in the Black Sea . In June 1917 , a Russian pilot captured by the Germans reported that a Russian airplane bombed and sank UB @-@ 7 on 1 October at position 44 ° 30 ′ N 33 ° 15 ′ E , near the Chersones Lighthouse . Authors Dwight Messimer and Robert Grant are each dubious of this claim , and the fate of UB @-@ 7 is still officially unknown . Among the fifteen men lost on UB @-@ 7 were the Constantinople Flotilla 's senior radio officer , and the first Bulgarian submariner lost during the war , a trainee from Vidin . 
