Guano is the Spanish word for dung. It's left by all animals, birds, and fish.
In UE terms, it's most often left, in staggeringly voluminous quantities by air rats and bats. It poses both slipping and air-quality hazards especialy when dried and inadvertently kicked into the air where explorers may breath it in. Apparantly the ammonia and methane produced by years' deposits of fermenting bat guano in caves and abandonments is enough to asphyxiate a person. As air rats and bats perch up high, there is also a danger one will evacuate itself on the unsuspecting explorer below. Don't look up if you hear cooing or sonar-clicking, and be sure to keep your mouth closed if you do look up when you hear those sounds.
The word 'Guano' is often used in situations with female explorers as it's more polite than saying, "That is one big pile of shit." or just because it's a funny word. ;)