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The Doomsub is a large, metallic, Plesiosaur-like submarine owned by King Bowser Koopa, that was only ever seen on The Adventures of Super Mario Bros. 3. Whereas the Doomship was built for travel above land, the Doomsub was created specifically for aquatic attacks. Although its origin was never explained, there is a good chance that the Doomsub was built by Kooky von Koopa, as he was the one who built the Doomship in the continuity of the cartoon series. Despite appearing prominently in two separate episodes, the Doomsub was never referenced in any other form of Mario Media.
The Doomsub has a number of special features and abilities. Like the Doomship, the Doomsub is equipped with special holes for Rocky Wrenches to hide in and attack from. It is also capable of launching Bob-ombs and Nasty Nets at unwanted targets. The Doomsub can also blast mysterious lasers, which have been shown to magically encase their target in a block of cement.
The Doomsub first appeared in the episode "The Ugly Mermaid", where Bowser, accompanied by Hip and Hop Koopa, used it to try and take over the underwater city of Mertropolis. Mario, Luigi, Princess Toadstool and Toad had pursued the Doomsub to Mertropolis, were they were bombarded by Rocky Wrenches. Mario and Luigi used the wrenches initially thrown by the Rocky Wrenches to retaliate against the Doomsub's attacks, and knocked it out of control by hitting the rotors. Bowser then activate the Doomsubs lasers, one of which hit Mario, and encased him in a block of cement, nearly drowning him.
Bowser soon announced that he would destroy Mertropolis within twenty minutes with the Doomsub if the citizens refused to hand over their city to him. With Mario too nervous to go back in the water, Luigi, Princess Toadstool and Toad attempted to stop the Doomsub on their own, only to get caught in Bowser's Nasty Nets. As the people of Mertropolis failed to surrender, Bowser dropped a Bob-omb from the Doomsub onto Mertropolis' dome, which blew up and started to flood the city. Mario got over his fear of drowning so that he could seal up the hole and rescue his friends. After the rescue, Mario and Luigi used one of the Nasty Nets to gather a school of Jelectros, which they used to electrocute the entire Doomsub, blasting it away from Mertropolis.
The Doomsub then reappeared in the episode "The Venice Menace". Mario and his friends first heard about the Doomsub, when a gondolier from Venice appeared in Desert Land, shouting about how a "sea monster" was attacking his home town. Although Mario and Luigi headed for Venice to stop the sea monster, they had no knowledge that it was actually the Doomsub. It turned out that Bowser and Kootie Pie Koopa were using the Doomsub to scare everyone out of Venice so that they could warp the city back to Dark Land and turn it into the "Kootie Pie Koopa Water Park".
Not long after Mario and Luigi arrived in Venice, they were spotted and chased around by the Doomsub. Using a motorboat, Mario and Luigi made it to dry land, where the Doomsub crashed and got a leak in one of its wall, which Kootie Pie had to fix with a small Fire Bro. To get back at the Mario Bros., Bowser deployed several Boss Bass and Chain Chomps to destroy the foundations of several buildings, sinking them. One of the buildings was the hotel that Mario and Luigi were sleeping in. However, Mario and Luigi evaded the minions that attacked them, changed into their Frog Suits, and went after the Doomsub. While Mario distracted Bowser and Kootie Pie by luring them into the Doomsub's engine room, Luigi pulled the Doomsub's recall lever, and recalled all the minions that had been unleashed on Venice. After returning to the helm, Mario pulled another lever, and left the Doomsub with Luigi, just as the entire sub warped itself to Desert Land. The impact of the warp damaged the Doomsub, which in turned ruined Bowser's plan, and sent Kootie Pie into tears.