A quick park and grab, please replace as found and BYOP.
Guano is a type deposit found in caves derived from the excreations of bats and in minor cases some birds. The organic depsoits decompose and form a series of acid fluids and gases that can interact with the minerals, sediments, and rocks present in the cave.
Visting the caves or stopping at the resturant for a tasty bite and cool drinkis is not required but highly recommended.