Mertropolis

Mertropolis is an underwater city protected by an oxygen-containing dome featured in the The Adventures of Super Mario Bros. 3 episode, The Ugly Mermaid. Bowser planned to take over the city using his Doomsub, renaming it "Kooptropolis", but his plans were foiled by the Mario Bros, who used a group of Jelectros against his submarine.

Although Mertropolis is oxygen-filled, the residents are fish with human legs. Since fish can't breathe oxygen, the people of Mertropolis wear water-filled fishbowls. (Strangely, the citizens panicked when Bowser flooded the place.) Notable residents of Mertropolis include its ruler King Mackerel and his daughter Holly Mackerel.

The town's name comes from a portmanteau of the words "merfolk" and "metropolis", a word for a big city.