Grand Koopa Canyon
From the Super Mario Wiki
Located in Car Land, the only way to cross the Grand Koopa Canyon, aside from jumping over it, appears to be a bridge connecting both sides of the canyon. Located at the bottom of the Grand Koopa Canyon is a large river. Additionally, a large hole, created from when King-of-the-Road-Koopa crashed into one of the canyon's walls, is also located in the Grand Koopa Canyon.