I can imagine this being true of a field that is no longer studied, although I can't think of any good example—the only unsatisfactory one that occurred to me is penmanship, but (a) that's not exactly a scientific discipline and (b) there are probably still people out there making strides in penmanship—but I find it hard to believe of any living discipline. In fact, I would argue that, if it is true of a discipline that its deepest mysteries were discovered in the past, then that discipline is effectively dead.