Amphibious assault submarine An amphibious assault submarine is a so far theoretical submarine equivalent of an amphibious assault ship . While such ships have been proposed in the past by both the U . S . and the Soviet Union , none has ever been built ( though at least one of the larger Soviet designs did start construction ) . Converted or standard submarines have often been used to transport small groups of soldiers or supplies , however . In the Soviet Union . The Soviet Union had some experiences using submarines to shuttle weapons , supplies and special forces into besieged areas or behind enemy lines in World War II , specifically during the siege of Sevastopol . Project 621 . After the war , in 1948 , Project 621 was proposed as a landing ship - transport submarine to set down troops behind enemy lines . With some 5 , 950 tons , the underwater giant would have been one of the largest submarines of its day . On its two vehicle decks , it was to carry a full infantry battalion of 745 troops , 10 T - 34 tank s , 12 trucks , 12 towed cannon , and 3 catapult - launched La - 5 fighter aircraft for air cover . The troops and vehicles meanwhile would be unloaded over a bow ramp when the submarine beached on the surface . Project 664 . While Project 621 was not built , follow - up designs got even larger and even more ambitious , though this was eventually to be their downfall as well . The additional capabilities , from being expected to do underway replenishment for other submarines to being capable of providing submarine rescue tended to delay the projects which were otherwise considered fully feasible . At least one , Project 664 , started construction at Severodvinsk in 1964 , but the mixed requirements of replenishment , transport , and mine - laying caused major complications in combination , even with the proposed nuclear propulsion . Project 748 . In the late 1960s , another proposal , Project 748 , neared construction , a nuclear - powered assault transport ( with less added requirements than previous designs ) of up to 11 , 000 tons . This submarine was to carry up to 20 amphibious tanks and BTR - 60P armored personnel carriers , and up to 470 troops , with the vehicles stored in double - deck hulls contained to both sides of the main hull , all three within an outer shell . The submarine was to be equipped with a torpedo armament of four bow 21 inch ( 533 mm ) torpedo tubes ( 18 to 20 torpedoes stored ) as well as anti - aircraft guns and surface - to - air missiles . It also was to have mine - laying capabilities . In the end , while the shipyards had already begun to prepare for the production of five submarines of the 748 type , this project was also scrapped in the early 1970s , as the manufacturing capacity was needed for new ballistic missile submarines . The ideas proposed during these projects helped drive the proposals for nuclear - powered merchant submarine s by Russian design offices in the 1990s . In the United States . The U . S . Navy has in the past also undertaken some preliminary design of submarine transports , but never the extent of the Soviet design efforts . In the 1950s a 10 , 000 - ton submarine was proposed , 220 meters ( 720 ft ) long , and with a beam of 38 meters ( 124 ft ) . It was to carry 2 , 240 Marines , landing them using ' amphibious flying platforms ' with an undefined propulsion method , supposed to move at 100 mph ( 160 km / h ) . However , many older submarines have been refitted during US naval history into transport submarines , mainly for use in covert operations , i . e . special forces deployment .