Oil Slick Shore

Oil Slick Shore is an anomalous level that can take the place of Hazy Maze Cave in certain copies. The course appears to be a combination of assets from Hazy Maze Cave and Dire, Dire Docks, commonly taking place on a lake of water with two shores on opposite sides. This level often has unique designs for the enemies that appear, such as contaminated creatures and industrial workers. A unique oil hazard can usually be found in this level, as well as a unique boss fight.

Layout
Oil Slick Shore, true to its name, starts out on a beach surrounded by grassy hills. The beach itself appears contaminated with oil slicks and contains enemies such as Goombas and Skeeters. These enemies tend to have black, oily bodies with yellow eyes, and/or appear to be drenched in the oil that covers the level. The water is a murky gray color, having spots of oil floating on the surface that damages Mario if he comes into contact with them. Throughout the water, aquatic enemies such as Bubs and Sushi commonly appear, possessing sickly green color palettes to match the polluted environment. Opposite to the body of water is an industrial area, although some reports of the level report that it is inaccessible, or not even present. It is often full of large machines that resemble drills and walkways with enemies. A section of the shore appears to have been dug out into a dry-dock, where Bowser's Submarine can sometimes be found. The drill-like devices appear to be the source of the pollution, as their exhausts spit oil and sludge out into the water. On the industrial shore are enemies such as Shy Guys and Motos, having hazmat suits and rusty bodies respectively. Behind the drills lies a huge hole with no visible bottom, which the Motos will try to throw Mario into if he gets caught. Underwater, there might lie broken pieces of excavating equipment, lying sideways after seemingly falling in. Normal fish swim in this area, as it is the one part of the water the pollution hasn't reached.

Boss Fight
In a reported mission, falling into the bottomless hole will teleport the player into an arena. It usually resembles the underwater cavern from Hazy Maze Cave but with oil-filled water and no sunlight coming from above. A version of Dorrie, commonly with oily skin and yellow eyes, appears and the fight begins. As the water is completely contaminated, the player cannot risk jumping off the island. A reported method to defeat the boss states that one must dodge Dorrie swinging its head down, and then perform a ground pound on its head to deal damage. After 3 successful hits, Dorrie will sink into the oil and leave behind a star for the player to collect.

Theorizing
Similarly to Salty Salty Shore, this level appears to be a combination of level assets thrown together by the AI. Considering the mechanics of the polluted water, it can be assumed that the concepts from this level were later reused in the game's sequel, Super Mario Sunshine.