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 it, they'll lose a balloon, and they'll have to be returned by a Lakitu.
There are four block towers located around the course. They're there to provide a means of evasion from opposing racers and their items. The course is slightly sloped inwards.
Big Donut reappears in Mario Kart 7 as a retro battle stage. It is mainly unchanged apart from the addition of glider pads.