The Mul Cavern is one of the last areas in Phoenotopia and Phoenotopia Awakening. It is a very dangerous area that Gale must traverse this location in order to reach the Ancient Crater. The Rocket Boots are also found here.
In Phoenotopia, the 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 are the Cave Beetles, which emerge from nests found throughout the caves. There are three different types of beetles, one explodes on death or on contact with Gale while another explodes into pools of acid on death. The nests they spawn from are also found in hard to reach places. Combined with the narrow spaces of the Cavern, these enemies can become overwhelming when a group of them start attacking Gale.
In Phoenotopia Awakening, the 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.