Underground Maze of Pipes
It first appears in the episode "Misadventure of Mighty Plumber", when Bowser and Mighty Plumber are trying to get to the Pipe Land Treasury. To do so, Mighty Plumber cracks open a pipe that spews water all over Bowser. A good portion of the episode takes place in the Maze, as Mario and Luigi try to stop Bowser and Mighty Plumber as they chase them through it.
A similar location appears in "Misadventures in Babysitting", when Kooky, Bully and Cheatsy Koopa toss a brat named Junior down a Warp Pipe to get rid of him. Junior is lost in a portion of the Pipe Maze, and is eventually rescued by Mario and Luigi, just as Kooky floods the entire area with lava.