Kinoko Cave is a custom track placed on a flooded mountaintop, where the water levels are decreasing. The players race on a mountain section with some mushrooms, before entering a cave in the bigger mountain, where they go through a cave with many optional mushrooms (although the track is significantly easier if you take the mushrooms, since by taking the suspended bridge section there's a huge chance of falling into the water) and an ultra shortcut possibility. The track's length is short, but it has a very high difficulty. It's a bit easier in RC2, because of removing glitches, etc.
Note: In v1-beta2, there is a special _d version of the track for Multiplayer. This is a temporary fix for Multiplayer lag until the model and KCL gets optimized.