Basically its what the main verb is, so it would be turns because it turns cool and dry.
My sisters friend and I=complete subject
sisters friend and I =simple subject
Answer: The catacombs are human-made subterranean passageways for religious practice. Many of the walls are aligned with human remains to the ceiling, and there is a small depression in the back wall. Overall, catacombs are dark, extensive, damp, and filled with piles of human bones.
