'Undine' was a German naval minelaying operation in the Baltic Sea (4/20 September 1939).
The minelayers Hansestadt Danzig and Tannenberg and the training ship Brummer, accompanied by the minesweepers M 75, M 84 and M 85 as well as the craft of Kapitänleutnant Walter Gemein’s 5th Räumboots-Flottille, departed during the early morning of 4 September loaded with about 700 mines to be laid at the southern end of the Great Belt. During the day this force was supplemented by the minelayer Preussen with 136 mines laid at the northern entrance of the Great Belt. These fields sank the first neutral victim of the war at sea, the 3,933-ton Greek freighter Kosti. The southern field was strengthened by Hansestadt Danzig and Preussen on 5/6 September.