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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. Despite appearing prominently in two separate episodes, the Doomsub is 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 appears in the episode "The Ugly Mermaid", where Bowser, accompanied by Hip and Hop Koopa, use it to try and take over the underwater city of Mertropolis. Mario, Luigi, Princess Toadstool and Toad pursue the Doomsub to Mertropolis, where they are bombarded by Rocky Wrenches. Mario and Luigi use the wrenches initially thrown by the Rocky Wrenches to retaliate against the Doomsub's attacks, and knock it out of control by hitting the rotors. Bowser then activates the Doomsubs lasers, one of which hits Mario, and encases him in a block of cement, nearly drowning him.
Bowser soon announces that he is going to destroy Mertropolis within twenty minutes with the Doomsub if the citizens refuse to hand over their city to him. With Mario too nervous to go back in the water, Luigi, Princess Toadstool and Toad attempt to stop the Doomsub on their own, only to get caught in Bowser's Nasty Nets. As the people of Mertropolis fail to surrender, Bowser drops a Bob-omb from the Doomsub onto Mertropolis' dome, which blows up and starts to flood the city. Mario gets over his fear of drowning so that he can seal up the hole and rescue his friends. After the rescue, Mario and Luigi use one of the Nasty Nets to gather a school of Jelectros, which they use to electrocute the entire Doomsub, blasting it away from Mertropolis.
The Doomsub then reappears in the episode "The Venice Menace". Mario and his friends first hear about the Doomsub when a gondolier from Venice appears in Desert Land, shouting about how a "sea monster" is attacking his home town. Although Mario and Luigi head for Venice to stop the sea monster, they have no knowledge that it is actually the Doomsub. It turns out that Bowser and Kootie Pie Koopa are using the Doomsub to scare everyone out of Venice so that they can warp the city back to Dark Land and turn it into the "Kootie Pie Koopa Water Park".
Not long after Mario and Luigi arrive in Venice, they are spotted and chased around by the Doomsub. Using a motorboat, Mario and Luigi make it to dry land, where the Doomsub crashes and gets a leak in one of its walls, which Kootie Pie has to fix with a small Fire Bro. To get back at the Mario Bros., Bowser deploys several Boss Bass and Chain Chomps to destroy the foundations of several buildings, sinking them. One of the buildings is the hotel that Mario and Luigi are sleeping in. However, Mario and Luigi evade the minions that attack them, change into their Frog Suits, and go after the Doomsub. While Mario distracts Bowser and Kootie Pie by luring them into the Doomsub's engine room, Luigi pulls the Doomsub's recall lever, and recalls all the minions that had been unleashed on Venice. After returning to the helm, Mario pulls another lever, and leaves the Doomsub with Luigi, just as the entire sub warps itself to Desert Land. The impact of the warp damages the Doomsub, which in turned ruins Bowser's plan, and sends Kootie Pie into tears.