This probably does not fully fit the criteria outlined by the Op because others have no doubt done the same thing in other parts of the world, but I was the first person to explore the underwater section of a cave we found on private property in southern Missouri in 1966. Kept hoping to find something exotic, but had to settle for some very pretty cave formations. As a college student and avid spelunker at the time, It was very adventurous knowing we were the first humans in that underwater cave system. Although we were experienced in caving and took every possible precaution, considering the dive gear we had available at the time, and the number of dives we made into that cave, I look back now and understand how lucky we were that one or more of us did not die.