Tephra Cave is a cavernous area in Xenoblade Chronicles. The cave itself is located inside the Bionis' right leg. It must be navigated at least two time throughout the game, the first time in order to reach the Cylinder Hangar and the second time to access Gaur Plain. It can initially be reached via the landmark named Tephra Cave Entrance at Colony 9.
This cave is the main path from Colony 9 to the upper regions of the Bionis such as Colony 6. A permanent squad of the Colony 9 Defence Force is within the complex.
Reyn was tasked with gathering Cylinders to operate a Mobile Artillery that was crashed into a house by soldiers from the Defence Force in the Residential District of Colony 9. The actual Cylinder Hangar appears to be part of a larger ship, which Shulk theorizes was made by ancient Homs.
After that the Mechon launch an assault on Colony 9, killing many Homs in the process including Fiora, giving a motive to the group for traversing the caves in order to reach Colony 6 where they hope to find a way to destroy Metal Face, the Mechon who led the attack. While in the cave, they reach some dead travelers. They decide to allow them to "return to the Bionis", leaving the corpse in a body of water. After this, they decide to rest. After Shulk wakes up, they are attacked by a group of Arachno, which take Reyn hostage. Shulk tries to catch up with him, scared because of a vision where he saw Reyn being killed by an Arachno Queen. After a long chase, he manages to catch up with Reyn just before he is killed by the said Arachno. However Shulk manages to unlock Monado Shield and saves Reyn. Together they manage to kill this beast and safely exit the caves, but not before theorizing that the spider was the reason why nobody from Colony 6 had reached Colony 9 for a while.
After the events in Mechonis Core, a new area of Tephra Cave opens up, which contains enemies of level 90 and up. The new path can be found at Vilia Lake, where there were previously rocks blocking the path. The new area also contains one of the Giant ruins.