 Although briefly a part of the Pola Flotilla at commissioning , UB @-@ 7 spent the majority of her career patrolling the Black Sea as part of the Constantinople Flotilla . The U @-@ boat sank one ship of 6 @,@ 011 GRT in September 1915 . In October , she helped repel a Russian bombardment of Bulgaria . She was considered for transfer to the Bulgarian Navy , but disappeared in late September 1916 before a transfer could take place . Her fate is officially unknown , but sources report that may have struck a mine or been sunk by a Russian airplane . 
