The USS Capelin departed for a patrol on November 17, 1943, but the sub and its crew of 78 men disappeared somewhere around the western Pacific Ocean, in the Celebes or Molucca Sea.
The USS Amberjack was carrying a crew of over 70 men when it disappeared. It is possible that the sub sank during an enemy attack on February 16. There is, however, no evidence of this.
Italian submarine Barbarigo, with contested claims of sinking American battleships, vanished off Bordeaux, France, in June 1943, likely due to enemy actions or mechanical failure.
The USS Kete was carrying a crew of 87 men when it vanished off the Ryukyu Islands around March 20, 1945. The cause for the disappearance remains unknown.
Estonia's Kalev-class submarines, Lembit and Kalev (1936), had Lembit survive and now reside in the Estonian Maritime Museum. Kalev vanished during its second World War II mission.
The sub was reported missing on February 1, 1944, though it's likely that it vanished about a month earlier, somewhere in the North Atlantic.
Orzel, escaped Estonia after the 1939 German invasion, reaching the U.K. Despite its celebrated escape, the Orzel and its 54-member crew vanished during a North Sea patrol in June 1940.
Dutch submarine O-13 vanished after leaving Dundee, Scotland, on a mission in June 1940. It's believed the sub was sunk by a mine, but the real cause remains unknown.
On December 22, 1944, the sub went out on its 13th mission, but never returned. Radio contact with the USS Swordfish was lost after January 2, 1945. No one knows exactly what happened.
The sub was last sighted on February 16, 1944. It's believed that the USS Trout vanished somewhere in the East China Sea.