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Submarines have since their introduction been key during naval warfare, and indeed played an important role during World War II. From Allied subs to German U-boats, naval fleets would not be the same without them. Though just as planes crashed and ships sank, many submarines and their crews also met their fate during the war. Some of them, however, simply disappeared without a trace.
In this gallery, we look back at some of the WWII subs that disappeared for unknown reasons, and still have never been found. Click on to discover them.
The USS Capelin (SS-289) was a Balao-class submarine, launched on January 20, 1943 and commissioned in June of the same year.
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.
One source states that the USS Capelin and its crew were lost at sea on December 1, 1943. Japanese attacks and naval mines have been put forward as theories for the disappearance of the USS Capelin, but thus far there are no conclusive answers.
The USS Amberjack was a Gato-class submarine, which was launched on March 6, 1942 and served on the Pacific front during World War II, starting in January 1943.
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.
The USS Grampus was a Tambor-class submarine that failed to return to Brisbane from a patrol on February 11, 1943.
It is believed that the USS Grampus suffered the same fate as USS Amberjack and was presumably attacked by Japanese destroyers.
The Barbarigo was an Italian Marcello-class submarine, first launched on June 12, 1938.
The Barbarigo served on numerous missions during World War II, but was eventually converted into a trade submarine. The Barbarigo was mainly used to carry materiel between Germany and Japan.
The Italian sub disappeared off the coast of Bordeaux, France, sometime between June 17-19, 1943. Whether it was attacked or it broke down, its fate remains unknown.
The USS Kete (SS-369) was a Balao-class submarine that disappeared after leaving the western Pacific island of Guam on March 1, 1945.
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.
The Kalev-class submarines were two British-built mine laying submarines used by the Estonian Navy.
Lembit was one of two Kalev-class submarines to serve and survive World War II. Her sister sub Kalev, however, was not so lucky.
EML Kalev never returned from its mission. The sub and its crew of 32 men disappeared somewhere around the Gulf of Finland in either October or November 1941.
The German Type VIIC submarine U-972 was a U-boat used by Nazi Germany's Kriegsmarine during World War II. Pictured is a Type VIIC U-570, which is identical to U-972.
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.
As for the reason, there is a theory that the U-boat was sunk by its own T5 acoustic torpedo, which may have followed a circular route. This hypothesis, however, has never been proven, so the reasons for the disappearance of the U-972 remain unknown.
The ORP Orzeł was a Dutch-built Polish submarine. When Germany invaded Poland in September 1939, the sub captain Jan Grudziński and his crew relocated to Estonia.
However, the Estonian authorities decided to capture the crew and remove the sub's weapons and navigation equipment. But Grudziński and his crew managed to flee and make it to the UK in 44 days, escaping the Nazis on the way.
Sadly, in June 1940, the Orzel was reported missing during a patrol mission in the North Sea. Various expeditions have been made to search for the wreck, but so far none has been successful.
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.
The USS Swordfish went out on patrols right after the December 7, 1941, attack on Pearl Harbor. The USS Swordfish was the first American sub to sink a Japanese ship during WWII.
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 USS Trout (SS-202) was a Tambor-class submarine that successfully served in the Pacific from 1941 to 1944.
The sub was last sighted on February 16, 1944. It's believed that the USS Trout vanished somewhere in the East China Sea.
The USS Grayling was a Sturgeon-class attack submarine that was active in the Pacific following the attack on Pearl Harbor.
The USS Grayling mysteriously disappeared sometime in late August or September 1943. No one knows what happened to the sub and its crew of 53 men.
Sources: (Grunge) (United States Navy) (Encyclopedia of British Submarines 1901-1955) (NATO)
See also: World War II: The day Japan surrendered
World War II submarines that disappeared without a trace
They have yet to be found
LIFESTYLE Mysteries
Submarines have since their introduction been key during naval warfare, and indeed played an important role during World War II. From Allied subs to German U-boats, naval fleets would not be the same without them. Though just as planes crashed and ships sank, many submarines and their crews also met their fate during the war. Some of them, however, simply disappeared without a trace.
In this gallery, we look back at some of the WWII subs that disappeared for unknown reasons, and still have never been found. Click on to discover them.