Big Donut is a Battle Stage in Mario Kart 64. The stage is in the shape of an enormous doughnut, as the name implies, and in the middle there is a large hole, full of lava. If a player falls into the lava, a balloon will automatically be lost.
There are four block towers located around the course. They're there to provide a means of evasion from opposing racers, and any, and all items they may throw at the player. The course is slightly sloped inwards.
This stage reappears in Mario Kart 7 as a retro battle stage. The players are now able to glide across the hole. Some lava will shoot up while gliding.