I am not a teacher.
If, as you write in the last sentence, the caves were not known to anyone until 1819, then that is when they were discovered. There are a number of mistakes in this text, though. Britisher is wrong. Only Nazi officers in B movies use this word, or old fashioned Americans. The second sentence is completely wrong. We'll call 'untill' a typo, shall we?
It isn't a very good text, I'm afraid.
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