Mul Cavern is one of the last areas in Phoenotopia and Phoenotopia Awakening. It is an underground cave system that is reached at the end of the Dread Lands. As one of the Forbidden Lands that were deemed too dangerous to travel to and sealed off by the Great Walls, it is a very dangerous area that Gale must traverse in order to reach the Ancient Crater. The Rocket Boots are also found here.
In Phoenotopia, Mul Cavern is a relatively short but mazelike area filled with bioluminescent life, including glowing mushrooms, plants, and Cave Toads. Spotted Lepista mushrooms can be found throughout the area. These mushrooms are consumable items that heal 15 HP but will shrink when taken out of the cavern.
When Gale reaches the entrance to the Cavern, Billy detects a distress beacon coming from deep inside. They find out that the beacon is coming from a space shuttle that has crashed inside centuries ago. Among the wreckage are the Rocket Boots, which allows Gale to hover over short distances.
The main danger of the Mul Cavern is the Cave Beetles, which are among the toughest enemies of the game. They spawn from nests that are in hard-to-reach places and aggressively attack Gale in large groups. The fact that Mul Cavern mostly consists of narrow spaces makes the Cave Beetles that much more frustrating to deal with when a bunch of them come after Gale.
In Phoenotopia Awakening, Mul Cavern is much larger and more labyrinthine than before, and even features three different biomes: the original mushroom-themed area, a section full of webs and nests, and a third unknown biome. It also has a new optional boss that can be found deep within the caves, a giant Cave Beetle queen named Ariadne.