Complex sentence: Great Onyx Cave is unusual in that it has not yet been connected to nearby Mammoth Cave; it is the only major Flint Ridge cave which has yet to be connected with the Flint Ridge Cave System, despite exploration efforts.
Split into simpler sentences: Great Onyx Cave is unusual in that it has not yet been connected to nearby Mammoth Cave. In fact, it is the only major Flint Ridge cave which has yet to be connected with the Flint Ridge Cave System, despite exploration efforts.