From the Super Mario Wiki
The Underground is a playing field made out of metal in Super Mario Strikers and Mario Strikers Charged. The stadium was funded by Wario and Waluigi. As its name implies, it is located several stories underground. In the store demo mode of Super Mario Strikers, The Underground is the only unlocked field.
According to the Japanese Super Mario Strikers site, it is located underneath "a certain city", possibly meaning Diamond City. The Capacity is 35,000; it is the Star Cup's special field.