From the Super Mario Wiki
Crater Field is a field made out of grass in Super Mario Strikers and later Mario Strikers Charged. It takes place in Yoshi's Island and the capacity is 42,000. It is the Super Mushroom Cup's special field. Instead of having fans sit in normal seats, they sit in glass-covered, egg-shaped sections. Many plants also grow in the stadium.
The structure is furnished with a heatproof material, which could be a protection against the warmth within the crater.