Stage 181 is an unused stage in Super Monkey Ball 2 that was designed to test certain level gimmicks. The stage can be accessed by using the Stage Select option in Debug Mode.
This level consists of the platform from Spinning Top (albeit without the top) that is connected to eight level concepts via short platforms with short walls.
In front of the starting platform is a sign with handwritten Japanese text. It is signed "by Jamad", which is the nickname of stage design director Junichi Yamada. This sign is identical to the one found in Debug Stage 183. There is also an 'A' button at the front of the stage that appears to be an early switch model; it does not appear to do anything.
The stage utilizes an alternative goal design that was dropped from the final game. It is layered over the final goal design, resulting in the ultimate appearance being a hybrid of the two.
The stage takes place in a magenta-colored void. The top of the area uses the water surface effect from Super Monkey Ball's Underwater world. There is light pink fog throughout the level that gradually changes to purple. The stage, like many unused stages, is primarily a brownish-green color, and the squares are much larger than on any finished stage's. The background music is the theme from Monkey Soccer.
Note: the cardinal directions below treat "North" as the direction the monkey faces at the start of the stage.
This is an early version of Warp. While most of the stage is the same, the ending is different. Only one goal is present, and it is located where the final version's green goal is; the platform it rests on is also slightly larger. There is a second goal floating in the distance that is impossible to reach without cheating.
This concept was never used in the final game. It consists of three sets of stairs, which intersect at odd angles and become thinner with each set. The last set ends in a basin holding the goal. There is also a swinging thin set at the junction between the first and second set that ends at an empty platform. Presumably, this set was intended to end at a warp goal.
This is an early version of Curve Bridge. The drawbridges are at the front of the stage instead of the back, where they are colored black and move in the same direction, unlike in the final. The goal floats in midair in the middle of the stage and can only be accessed by cheating or by making exact collisions with moving parts elsewhere in the stage.
This is an early version of Sieve. This consists only of the long stretch at the end of the final version of the stage. Unlike in the final version, there are two sieves that move in tandem. The two sets of larger rings and the circular catwalk are not present here. Both the stretch and the platform at the end (which is slightly different from the final's) are solid cyan.
This is an early version of Zigzag Slope. Unlike the final version, this floor is symmetrical: Both paths are as steep as the final's right-hand side.
This is an early version of Combination. The stage is mostly the same, but the level is always at fast-forward speed. The set of squares with the goal is color-coded red. Interestingly, it is possible for the squares to clip through the platform leading to this section.
This is an early version of Mad Rings. There is only one ring, which moves at the same speed as the final's second ring. Presumably, the green and red goals, and thus more rings, were added later.
- This stage's model names suggest possible prototype names for two levels, as well as the unused concept: