Just recently the wrecks of two WW I submarines were found in the Baltic:
German U-26 found off the coast of Finnland which sank after it hit a Russian mine 30 August 1915. This U-boat is best known for sinking the Russian crusier Pallada in 1914. The Pallada's wreck was found in 2012.
Russian Akula found off the Estonia coast where it sank after hitting a German mine 30 November 1915.
Both submarines were lost with all hands and both were cruising on the surface when they hit mines.
I hope this is of some use or interest.