Infinite Caves of the Shroom Goblins

From the introduction: The Infinite Caves exist on their own little sub-plane (thus explaining the horrible mapping confusion that will occur when the characters start exploring). Access is by a magic portal, and for all we know there may be nearly infinite numbers of these portals on the prime material plane, but they all enter the caves in the same manner, dumping the party out in between the mirrors in area 10 of the caves.