I had typed the word "quite," but then I checked the hover hints, which say, "considerably, greatly." I changed the word to "considerably," and the program said it was wrong and was supposed to be "quite." I will report that the hover answers are incorrect here. April 1, 2020. (No, I am not trying to do an April Fool's Day joke.)
"The lake in the mountains is considerably deep" is not correct english. "considerably" is used in comparisons: "the lake in the mountains is considerably deeper" is correct sentence. especially, notably, particularly, quite, very, rather, really would all be OK translations if 상당히 is a generic intensifier
Very good point.
"Considerably" is a quantifier for comparatives (verbs and adverbs).
I guess to use 상당히 as considerably/significantly, the Kor sentence needs to change slightly, e.g.
산 속에 있는 호수는 "이전보다" 상당히 깊어요 = The lake in the mountains is considerably deeper "than before".
산 속에 있는 호수는 상당히 깊어졌습니다. The lake in the mountains has "deepened" considerably. (To deepen/become deeper -> comparative verb)
As it stands 상당히 = quite, fairly, pretty, etc.. as you said.
The English translation here is quite awkward. I am not sure but is mountain and mountains the same in both plural and singular form? If it's only one then the preposition should be on I believe. If the Korean is talking about multiple mountains then the Korean should be different and that therefore changes the preposition to in instead of on.