From the Super Mario Wiki
Big Donut is a battle course in Mario Kart 64. As its name implies, the course is in the shape of a large donut. In the center hole of the donut is a pool of lava. If the players fall in it, they lose a balloon, and a Lakitu brings them out. There are also four rectangle-shaped blocks located around the course that provide a means to evade opposing racers and their items. The course is also slightly sloped inwards.
Big Donut reappears in Mario Kart 7 as a retro battle course. In the game, the stage is given new elements. There are jumping ramps so that players can use the gliders to fly over the pool of lava. The Koopa Clown Car appears floating in the center of the course which makes air currents so players can glide even higher. Lava springs often rise from the pool to plummet the players while gliding and let them fall into the lava.
Instruction manual description
This map is in the shape of a big donut. Because there is no guardrail along the inside, should you make a mistake, you will dive into the lava. Use the four walls to hide and ambush your opponents.
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