For those who have not completed games in the Xenoblade Chronicles series, this page contains spoilers regarding the plot. Discretion is advised.
The Third Lung is a location in Xenoblade Chronicles. It is located on the lower level of the Bionis' Interior, and is one of the three Lungs in the area. It is accessed from the Terminal Trachea at the far west end of the area, which leads south from the lung to the entrance from Satorl Marsh. At the east end there is a pod of stomach acids, in which there is a geyser which takes the party up to the passage leading to Upper Trachea and the exit to Makna Forest. Next to this geyser is a Nopon, the sole NPC found in the entire area, who tells them how they can use the geyser.
The first time the location is visited, it is empty, with no enemies or collectables. However, on when the party returns later in the game, the area will be filled with Selua enemies like the other two lungs, but collectables will only start appearing once the party moves into the Bionis' Interior via the Junks by speaking to Miqol. The party discusses the fact that the area seems much more alive on this visit than their first visit. This is due to the fact that Bionis has begun to awaken at this point, and thus the interior is filled with the organisms that live in any living being, albeit much bigger organisms due to the size of the Bionis.