10. George Washington Class By the grim logic of the Cold War, submarines armed with strategic nuclear missiles did much to keep the peace. Hidden under the ocean and extremely difficult to detect, the nuclear-powered ballistic missile submarine (SSBN) was the perfect deterrent. A potential adversary knew that even if a surprise nuclear attack wiped out land-based missiles and bombers before they were launched, the SSBNs (or "boomers") would survive to retaliate. The world's first SSBN was the USS George Washington, first ordered in 1957 and commissioned in 1959. Earlier Soviet subs had carried nuclear-armed ballistic missiles, but they were diesel-powered boats with limited endurance. The George Washington's nuclear propulsion enabled it to remain underwater for months without surfacing. Each of the five boats in the George Washington Class carried 16 Polaris missiles, giving a single submarine the capability to devastate an opponent's heartland. 9. Type XXI U-boat The Allies were fortunate that the Type XXI U-boat arrived too late to see combat. Had it been deployed before the end of war, it could have had a devastating impact on the Battle of the Atlantic. The Type XXI had numerous advanced features for its time, including high-capacity batteries that enabled it to remain underwater for days, a streamlined hull, and a snorkel to recharge the batteries while underwater. With an underwater speed of 17 knots, it could actually outrace most surface warships. 8. Typhoon Class Soviet-built Typhoons are the largest submarines in the world, weighing in at 48,000 tons (a U.S. Ohio-class ballistic missile sub weighs less than 20,000 tons, while an Arleigh Burke-class destroyer is only about 9,000 tons). Despite its size, the six Typhoons that were built were surprisingly quiet and hard to detect. They carried 20 SS-N-20 (NATO code name "Sturgeon") ballistic missiles equipped with multiple nuclear warheads, as well as anti-ship guided missiles and torpedoes. The boats featured multiple pressure hulls for greater strength. 7. Sentoku Class A submarine that's an aircraft carrier seems like mating a fish and an elephant, but that didn't stop several navies from trying. The Imperial Japanese Navy's I-400 Sentoku Class boats of World War II were 6,500 tons, almost three times the size of U.S. Gato Class subs, and about the same displacement as a U.S. George Washington-class nuclear-powered missile sub from the early 1960s. The three Sentoku boats each carried three torpedo-equipped M6A Seiran floatplanes that would be launched by catapult, and then ditch in the water upon their return. 6. X-Craft While many of the most famous submarines were giants, at the other end of the sub spectrum were the midget submarines. Britain's X-craft were used for special attack missions in heavily defended harbors that were impervious to conventional submarine attack. Their most famous raid was in 1943, when X-craft damaged the German battleship Tirpitz in a Norwegian harbor. The tiny 27-ton submarines were towed by larger boats to the target area, where they were cast off to make their way to the target. The crew would plant explosive charges before returning to the mother sub. 5. USS Nautilus The USS Nautilus was the world's first nuclear-powered submarine. Until the Nautilus, submarines were powered by diesel engines that could not be used underwater. Subs could either expose themselves on the surface when using their diesels, or run submerged on batteries that had limited power. But a nuclear-powered submarine could spend its entire voyage submerged and hidden. In 1955, for her maiden voyage, the Nautilus traveled 1,100 nautical miles - the longest submerged cruise in history at that time. 4. T Class Known also as the Triton Class, these boats formed the backbone of the British submarine force during World War II. Displacing about 1,500 tons, T Class subs packed a heavy punch of 10 torpedo tubes. However, they all fired forward, compared to other subs that could fire torpedoes with both bow and stern tubes. T-class boats exacted a heavy toll of Axis shipping supplying Rommel's Afrika Korps, but suffered heavy losses in the narrow, shallow waters of the Mediterranean. 3. Gato Class American Gato Class submarines were the bane of Japanese merchant ships during World War II. Fast, well-armed and with good range, they were well-suited for the undersea war against Japan. Gatos displaced about 1,500 tons, and were armed with six bow and four stern torpedo tubes. They could travel 20 knots on the surface and nearly 9 knots submerged. 2. Seawolf Class At more than $4 billion apiece, Seawolf Class submarines were the most expensive in history. But they were designed for a mission that brooked no failure; stalk and destroy Soviet ballistic missile subs before they could launch their weapons. Designed to be extremely fast and extremely quiet, the high price tag and diminished Soviet submarine threat with the end of the cold war caused the program to be cancelled after the first three boats were delivered. One of them, the USS Jimmy Carter, has since been converted into a spy sub. 1. Type VII U-Boat Perhaps the ultimate symbol of the deadly underwater predator, the Type VII U-boat may be the most famous submarine class in history. It was also the most numerous, serving as the workhorse of the German submarine forces in World War II. At around 900 tons displacement, and armed with five torpedo tubes, the Type VII was smaller than the big American fleet boats, but it came perilously close to winning the Battle of the Atlantic. But the cost was frightful; of 1,100 U-boats constructed in World War II - including 700 Type VII's - more than 800 were lost. More than 75 percent of U-boat crewmen perished.