There are currently two Mushroom Forest Caves known to exist:
The first cave can be found inside the stem of a very large Tree Mushroom within the Northwestern Mushroom Forest, towards the Northwest of the map, with many entrances both at the top and the bottom.
The second one is situated in the Northeastern Mushroom Forest and is connected to the Bulb Zone Caves. The entrance to this cave can be found in the Bulb Zone. This cave is particularly large (larger than the northwestern cave) and is made up of long, winding tunnels that lead to and from a large central cave found inside.