The Piranha Pit

Not to be confused with Piranha Valley.

The Piranha Pit is an anomalous level found in Super Mario 64 DS. It was originally meant to be unlocked by linking the game with Wow! Waluigi via the cancelled StarPass, but it can appear without it due to the A.I. spawning it on it's own. It's described as an area similar to Goomboss Battle, being a massive forest, with several thorny vines around the level that damage the player upon contact. It's boss, the Big Piranha Plant, was meant to be the boss that unlocked Waluigi upon defeat before it's removal alongside the cancellation of Wow! Waluigi. It can reportedly be found in the Negative World, but several other locations have been documented.

Description
The Piranha Pit is generally described as a forest very similar to that of Goomboss Battle, except the skybox doesn't have the fog and the poisonous gas are replaced with this level's vines. As one would expect, Piranha Plants are this level's primary enemy, but Goombas and Lakitus are also seen. The geography of the level gradually goes upwards, leading to the pipe to the boss fight and then one of the level's Power Stars.

Power Stars
Excluding the boss fight, this level has 2 stars: One is a reward for collecting 8 red coins scattered around the level, and the other is for reaching the very top of the level on the highest tree stump. Notably, the path to said tree stump has more enemies and hazards than the other parts of the level, making it more challenging to reach.

Boss Fight
This level's boss is the Big Piranha Plant. The arena is a spherical bowl, dipping down in the center to the Big Piranha Plant. The Big Piranha Plant, as one would guess, is a large Piranha Plant. It usually has the same design as a regular Piranha Plant excluding size, but it can reportedly appear purple, black, and/or yellow, likely due to it's ties to Waluigi. They attack by using their roots, which are actually the vines seen throughout the level, to swipe at the player, as well as just lay them on the ground as a stage hazard, spawning regular Piranha Plants, and biting at the player. They have to be hit on their head, either by being ground pounded or being hit with Yoshi's eggs. Upon being defeated, they drop a key with an upside-down L that would unlock Waluigi as a playable character.