The most difficult sub­marines riddles

Nuclear reactor requires no oxygen for combustion and emits no exhaust gas. It gives the submarine the ability to operate submerged at high speeds, for unlimited periods, dependent only on the endurance of its crew. Nuclear power revolutionized the submarine, finally making it a true "underwater" vessel, rather than a "submersible" craft.

Dmitriy Donskoy (TK-208) is a Russian Navy nuclear ballistic missile submarine, designated Project 941 Akula class (NATO reporting name Typhoon). With the decommissioning and scrapping of her Typhoon sister boats (TK-202, TK-13, Simbirsk, Arkhangelsk, Severstal, and TK-210), she is the largest submarine in the world in active service.

Surcouf was the largest French submarine cruiser having served during the Second World War. The boat was lost during the night of 19 February 1942, in the Caribbean Sea, possibly after colliding with an American freighter. Surcouf was named after the French privateer Robert Surcouf. It was the largest submarine built until surpassed by the first Japanese

Dudley Walker "Mush" Morton was a submarine commander of the United States Navy during World War II. He was commander of USS Wahoo during its third through seventh patrols. Wahoo was one of the most-celebrated submarines of World War II, sinking at least 19 Japanese ships, more than any other submarine of the time. Morton and Wahoo disappeared in 1943 during a patrol near La Perouse Strait, and he was ultimately declared deceased three years later.

Kaitens had a very limited diving depth, which in turn limited the diving depth of the host submarine. This is one of several factors blamed for the very poor survival rate of submarines armed with kaitens: eight submarines being lost for the sinking of only two enemy ships and damaging several others.

On 14 April 1945, while U-1206 was cruising at a depth of 200 feet, misuse of the new toilet caused large amounts of water to flood the boat. The leak flooded the submarine's batteries causing them to release chlorine gas, leaving him with no alternative but to surface. Once surfaced, U-1206 was discovered and bombed by British patrols, forcing Schlitt to scuttle the submarine. One man died in the attack, three men drowned in the heavy seas after abandoning the vessel and 46 were captured.

On the night of 31 May – 1 June, three Japanese Ko-hyoteki-class midget submarines, each with a two-member crew, entered Sydney Harbour, and attempted to sink Allied warships, ultimately killing 21 sailors.