The Flooded Plains

The Flooded Plains is the name given to a simple anomalous level that can reportedly appear in Super Mario 64. The level reportedly has very little assets, making it very likely that this level was scrapped earlier in development or created through Dynamic Level Arrangement. The Flooded Plains is described as a grassy plains area underwater with the Wet-Dry World skybox. The level can reportedly be found in place of any water level.

Description
The Flooded Plains is a very basic, cube-shaped level made of grass covered entirely in water with the Wet-Dry World skybox. Notably, the top of the water has supposedly been located that leads to a new level, such as the supposed City World level taking place in the Wet-Dry World skybox, though these reports have been discredited. Other reports of a supposed level or sub-area have been found, with descriptions such as an area inside of the grass cube or an island or above the water. Enemies such as Bubs or Bloobers have been reported in the level, though no stars have been reported.

Theorizing
Two different theories have been proposed explaining the simplistic nature of the Flooded Plains. The first one simply states that the level was a placeholder for an area that was never finished, or that the grass cube was a placeholder model for the level before it was scrapped. The second one states that the Flooded Plains was created by the Personalization AI through Dynamic Level Arrangement, explaining the simplistic nature of the level and reuse of assets such as the skybox.