Underground Maze of Pipes
From the Super Mario Wiki
It first appeared in the episode "Misadventure of Mighty Plumber", when Bowser and Mighty Plumber were trying to get to the Pipe Land 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.
A similar location 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 a portion of the Pipe Maze, and was eventually rescued by Mario and Luigi, just as Kooky flooded the entire area with lava.