Underground Maze of Pipes
From the Super Mario Wiki
The Underground Maze of Pipes is exactly what it sounds like: a maze of Warp Pipes that is underground. It is located beneath Pipe Land, and it is only seen on The Adventures of Super Mario Bros. 3. The pipes in the maze lead to numerous locales in the Mushroom World.
It first appeared in the episode "Misadventure of Mighty Plumber", when Bowser and Mighty Plumber were trying to get to the Pipe Land Coin Treasury. To do so, Mighty Plumber cracked open a pipe that spewed water all over Bowser. A good portion of the episode took place in the Maze, as Mario and Luigi were trying to stop Bowser and Mighty Plumber as they chased them through it.
It later appeared in "Misadventures in Babysitting", when Kooky, Bully and Cheatsy Koopa tossed a brat named Junior down a Warp Pipe to get rid of him. Junior was lost in the maze, and was eventually rescued by Mario and Luigi, just as Kooky flooded the entire maze with lava.