The context of a word refers to other words and phrases that surround a word and help the reader understand a difficult or unfamiliar word. Int he case of the word "subterranean" this is used to describe the system of rivers a group of scientists found after drilling or perforating the ground and therefore it is expected based in this context that the word "subterranean" means under the ground or below the surface as it is a feature of systems that can only be found if you drill into the ground. Thus, the correct answer is "under the surface".
